US President Barack Obama warned of the dangers of nuclear terrorism in his opening remarks Friday at the Nuclear Security Summit in Washington.

"There's no doubt that if these madmen ever got their hands on a nuclear bomb or nuclear material, they most certainly would use it to kill as many innocent people as possible," Obama said.

Speaking to leaders representing some 50 countries who gathered for the fourth such summit, Obama said that protecting nuclear material represented "a 21st-century security challenge that no one nation can solve alone."

He called nuclear terrorism the greatest threat to the world's collective security.

Noting that "no terrorist group has succeeded thus far in obtaining a nuclear weapon" and that international efforts to date had "measurably reduced the risks" of nuclear material falling into the wrong hands, Obama pointed out the threat nevertheless "persists."

Al-Qaeda had sought nuclear material, and the attackers behind last week's Brussels bombings had videotaped nuclear plant employees, Obama said.

He pointed out that Islamic State forces have used mustard gas.

The closing hours of the two-day summit would concentrate on further collective measures to secure nuclear materials.

More than a dozen nations have removed all of their highly enriched plutonium and uranium, Obama said. Japan is currently removing a half ton of highly enriched nuclear material, the largest such project in history.

Obama said there was still 2,000 tonnes of nuclear material around the world, and "not all of this is properly secured." Just a small amount - "the size of an apple" - could kill and injure hundreds of thousands of people, he warned.

On the sidelines of the summit, China has agreed to work with the United States to intensify efforts to prevent smuggling of nuclear material across borders.

Speaking Thursday in Washington, Li Wei, China's deputy head for customs control, said that China will "through cooperation, improve its ability to prevent the illegal trafficking of nuclear and other radioactive substances."

Russia chose not to attend the summit.

Earlier Friday, Obama met with leaders of the P5+1 group, which negotiated the Iran nuclear deal. He thanked all the nations who participated in bringing the agreement to fruition.

"Thanks to this deal, we have seen real progress," Obama said.

He noted that it will "take time for Iran to reintegrate into the global economy, but Iran is already beginning to see the benefits of this deal."

Latest news

Syrian opposition rules out future role for President al-Assad

The Syrian opposition said Friday it would not accept any role for President Bashar al-Assad in the future of the war-torn country, reacting to a recent US shift saying that removing al-Assad is no longer a priority for Washington.

Russian Army integrates breakaway forces of Georgian province

Parts of the small fighting forces of the Georgian breakaway province of South Ossetia have been placed under Russian military control, Russian Defence Minister Sergei Shoigu said on Friday.

Czech Republic's Pilsner Urquell beer is now Japanese

Japanese brewing company Asahi completed its takeover of the Czech brewery Pilsner Urquell on Friday, Asahi said in a statement.

Judge approves 25-million-dollar settlement of Trump University case

A US district judge on Friday approved a 25-million-dollar settlement of lawsuits and state fraud allegations against Trump University, the US president's now-defunct business venture.

Former Thai premier Thaksin to junta on reconciliation: 'Cut me out'

Former Thai prime minister Thaksin Shinawatra on Friday announced that he is not interested in the junta-led reconciliation process, three days after the junta handed him a half-a-billion-dollar tax bill for his past business deal.

Dalic: We welcome possible deal between Agrokor and banks

The government welcomes the possibility of an agreement being concluded between the Agrokor food company and creditor banks, and the bill on vitally important companies is not a fallback plan but the result of the government's care for the overall economic and financial stability of Croatia, Deputy Prime Minister and Minister of Economy Martina Dalic told a press conference in Zagreb on Friday.

Croatia, China sign action plan for cooperation in agriculture

The Croatian and Chinese ministries of agriculture on Friday signed an action plan for cooperation in the field of agriculture for the period 2017-2018, the Croatian ministry said in a statement.

ZSE indices up, Agrokor shares in focus of investor interest

The Zagreb Stock Exchange (ZSE) indices on Friday rose by more than 1.8%, with stocks of the Agrokor food and retail concern being in the focus of investor interest again.

Berlin police defend handling of Berlin market attacker

Berlin police defended themselves on Friday against accusations that they stopped surveillance on Berlin Christmas market attacker despite knowing in June 2016 he was dangerous.

Croatia, creditors tailor emergency measures to save tottering giant

Croatia's tottering retail and food giant Agrokor reached an agreement with its creditors, putting its debts standby and allowing it to continue working during emergency restructuring, the Croatian branch of Austria's Erste Bank said Friday.

Agrokor's creditors say standstill agreement to go into force today

A standstill agreement regarding the Agrokor concern's existing financial obligations to banks will take effect on Friday, additional capital will be injected into the concern in the coming days and the concern will be actively restructured, which includes a change of its management, it was said on Friday after a meeting between Agrokor's suppliers and creditor banks.

Palestinians, UN slam Israel's new settlement plan

Palestinians, Israeli activists and the UN lambasted the government of Israeli Prime Minister Benjamin Netanyahu on Friday, a day after it gave the go-ahead for the first new West Bank settlement in a quarter of a century.

South Sudan rebels release three abducted foreign oil workers

South Sudanese rebels have released three foreign engineers they abducted in early March in the oil-rich Upper Nile region, Foreign Affairs Ministry official Mawein Makol Arik said on Friday.

Turkish opposition: Imprisoned party chief has gone on hunger strike

The head of Turkey's pro-Kurdish opposition party has launched a hunger strike from prison.

European leagues threaten Champions League schedule clashes

The European Professional Football Leagues (EPFL) on Friday threatened schedule clashes on Champions League matchdays in an ongoing dispute with the governing body UEFA.

Danish court revokes citizenship of IS volunteer

A Danish appellate court on Friday stripped a man of his Danish citizenship for volunteering to fight for the extremist Islamic State in Syria.

Banks and Agrokor agree on key elements of standstill agreement

Member banks of the coordinating committee of financial creditors and representatives of the Agrokor food company have in principle agreed on key elements of a standstill agreement, which is expected to be signed later today, announcing changes in the company's management team, Erste Bank said in a statement on Friday afternoon.

Syrian man on trial in Sweden; mosque attack labelled terrorism

A Syrian man went on trial Friday in the southern Swedish city of Malmo, charged with terrorism and arson after an attack last year on a building used as an assembly hall by Shiite Muslims.