The United States and Russia have agreed to the terms for a Syrian ceasefire to begin Saturday, the State Department said.

The agreement, which does not apply to Islamic State and other UN-designated terrorist groups, would begin at midnight local time Saturday (2200 GMT Friday).

"I am gratified to see the final arrangements concluded today for a cessation of hostilities in Syria and call on all parties to accept and fully comply with its terms," Secretary of State John Kerry said.

"If implemented and adhered to, this cessation will not only lead to a decline in violence, but also continue to expand the delivery of urgently needed humanitarian supplies to besieged areas and support a political transition to a government that is responsive to the desires of the Syrian people."

The Syrian government and opposition groups must indicate their willingness to abide by the ceasefire agreement by noon Friday.

Late Monday, the Syrian opposition's Higher Negotiations Committee said it is still engaged in consultations on the terms for cessation of hostilities, proposed by Russia and the United States.

The committee said its agreement is conditional on there being international guarantees that Russian and Iranian forces will respect the truce and on the implementation of humanitarian measures including an end to sieges, artillery shelling and air raids.

US President Barack Obama and Russian President Vladimir Putin spoke by telephone ahead of the announcement.

Obama "welcomed that an understanding was reached between the United States, Russia, as well as other partners in the International Syria Support Group," the White House said.

He stressed the priority was now to ensure "positive responses" from the Syrian regime and opposition group and be sure the deal is implemented.

Putin said in televised remarks in Moscow that the agreement is a "real step that can stop the bloodshed" in Syria and sets an example in multilateral efforts to combat terrorism.

Russia will hold talks with the Syrian government to implement the truce, he said.

The US acknowledges implementation will be difficult.

"We know that there are a lot of obstacles, and there are sure to be some setbacks," White House spokesman Josh Earnest said.

Under the agreement, the US and Russia will exchange information to prevent parties to the ceasefire from being attacked by the US-led anti-Islamic State coalition, the Russian military, the Syrian military or opposition groups.

The US and Russia will establish a "communication hotline" to exchange information after the ceasefire is in place.

Airstrikes against Islamic State targets will continue, and the sides will share information about territory controlled by the terrorist militia.

Syrian President Bashar al-Assad, meanwhile, called parliamentary elections for April 13, state news agency SANA reported. The rubber-stamp Damascus parliament is nearing the end of its four-year term.

The terms of the ceasefire came three days after a temporary cessation of hostilities, proposed by world powers meeting early this month in Munich, was originally due to go into effect.

The cessation was not evident on the ground, however, with the Syrian Observatory for Human Rights watchdog noting that Sunday attacks marked the deadliest one-day bombings since the Syrian uprising erupted in 2011.

The suicide attacks in Damascus and Homs, claimed by Islamic State and targeting Syrian government-held territories, killed 184 people, in the largest single-day death toll from such blasts, a monitoring group said Monday.

Statements in the name of the Islamic State movement, posted on social media, claimed responsibility for both attacks. The claims, which could not be verified, were consistent with previous statements by the group, including claims of responsibility for three other deadly attacks since December in the same areas.

Islamic State and allied jihadist groups, meanwhile, attacked the sole supply route for the government to Aleppo city in the north, cutting off a key road near the town Khanasser to the south-east.

Aleppo, the second largest city in Syria, is divided between rebels in the east and regime forces on the western side. The government, backed by intense Russian airstrikes, is closing in on the rebels and experts predict the eastern part of the city could fall under siege.

"Regime forces have launched a counterpart and are advancing to take the vital road back," Abdel Rahman, head of the Syrian Observatory for Human Rights, told dpa.

UN Secretary General Ban Ki-moon welcomed Monday's announcement, noting that it was a "significant step forward" toward implementation of a UN Security Council resolution laying out the terms of the peace process, said Stephane Dujarric, Ban's spokesman.

"It is a long-awaited signal of hope to the Syrian people that after five years of conflict there may be an end to their suffering in sight," Dujarric said.

The Syrian war, which started in 2011 after al-Assad's forces launched a brutal crackdown against protests, has cost 250,000 lives and driven half the country's population from their homes, according to UN estimates.

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