US police faced renewed violence during the night in ongoing unrest over race-related killings as President Barack Obama prepared to cut short a diplomatic trip to Spain on Sunday to return to the United States to deal with the situation.

In the city of Saint Paul, Minnesota, police and marchers from the Black Lives Matter movement clashed during a protest about the shooting to death of Philando Castile by police in the area on Wednesday night.

"At least five ... officers were injured by people throwing rocks, bottles, fireworks and bricks. None seriously," the Saint Paul Police Department said on Twitter.

The Star Tribune newspaper posted images of a crowd facing off with officers on an interstate highway, which was blocked for hours during the clashes. Police used smoke bombs to disperse the group.

Obama arrived in Spain late Saturday at Torrejon de Ardoz military base outside Madrid. He was greeted by King Felipe VI, who escorted Obama down a red carpet lined by an honour guard.

The reception by the Spanish monarch shortly before 11 pm (2200 GMT) was exceptional, Spain's royal house said, adding that it was proof of the close relationship between the two countries. A number of Spanish government ministers, Spain's ambassador to the US and dignitaries were among the party that greeted the US president.

Obama embarked on the first visit by a US president to Spain in 15 years after taking part in a NATO summit in Warsaw. He opted to shorten his visit by one day after an attack on police in Dallas, Texas.

Before landing, Obama telephoned Texas Governor Greg Abbott to offer condolences on behalf of the American people for the five police officers who were killed in the attack on Thursday night. Seven other officers and two civilian bystanders were wounded.

The White House said Obama would leave Madrid on Sunday evening after meeting with King Felipe and Prime Minister Mariano Rajoy. He was to make a stop at a naval base in Rota near Cadiz in southern Spain before departing for Washington. Plans to stop in Sevilla and meet with young people in Madrid were canceled so that Obama could travel back to the US and prepare for his visit to Dallas.

At a news conference before leaving Poland, Obama called on Americans to unite against acts of violence and "not let the actions of a few define all of us."

The gun-fuelled violence saw two black men fatally shot by police on Tuesday and Wednesday, followed by the shooting in Dallas that targeted police officers at a peaceful rally protesting violence against the black community.

"This has been a tough week, first and foremost for the family of those killed, but also for the entire American family," Obama said. "As painful as this week has been, I firmly believe that America is not as divided as some have suggested."

The first of the three shootings occurred on Tuesday, when Alton Sterling, 37, was tackled and shot by police after selling CDs outside a convenience store in Baton Rouge, Louisiana.

The next day, Castile, 32, was fatally shot at a routine traffic stop in suburban Minneapolis, Minnesota. Castile's fiancee recorded the aftermath using Facebook Live as her boyfriend bled profusely while the officer continued pointing his firearm at him.

On Thursday, as demonstrators with the Black Lives Matter movement marched in Dallas, a black US military veteran opened fire on police who were guarding the march, killing the officers.

Police identified the lone gunman as Micah Xavier Johnson, 25, saying he told officers before he died that he was angry about the killing of black men by police officers and wanted to kill police.

Police said they killed Johnson with a bomb carried by a robot into the parking garage where the assailant had taken cover. A search of Johnson's home turned up bomb-making materials, ballistic vests, rifles, ammunition and a personal journal of combat tactics.

Obama described the gunman as "demented" and said though Americans felt sorrow, anger and confusion over the shootings, they should unite against the incidents and reject them.

"This is not who we want to be as Americans, and that serves as the basis for us wanting to move forward in a constructive and positive way," Obama said, adding that there was not "enormous polarization" in the US.

Police nevertheless have increased security in cities across the country, and in a sign of the tense situation that continues in Dallas, police placed their headquarters on alert Saturday while they searched for a suspicious person in a nearby parking garage.

Special forces were activated, and security in the area was increased, according to news reports.

The department said later on Twitter the search turned up no suspicious person.

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