U.S. Secretary of State John Kerry said Sunday that the deadly mall siege in Kenya had highlighted the gravity of the threat posed by Somali militants.

Kerry said he had spoken with Somalia's foreign minister and ambassador to Washington earlier in the day about the attack on a Nairobi shopping mall.

Five Americans were among those wounded when Somali militants stormed a Nairobi shopping mall, according to a statement by the U.S. State Department on Sunday.

"A number of American citizens were present in Westgate Shopping Mall at the time of the attack. Five American citizens are among the injured, but we have no reports of any American deaths," State Department spokeswoman Marie Harf said in a statement.

Three Britons have been confirmed dead in an attack on a Nairobi shopping mall by Somali Islamist militants and the British toll is likely to rise, the Foreign Office in London said Sunday.

"Three British nationals are confirmed dead. The next of kin of those nationals have been informed," a ministry spokesman said. "The number of British nationals confirmed as killed is likely to rise as further information becomes available."

A lawyer for Kenyan Vice President William Ruto asked the International Criminal Court on Sunday to adjourn his trial for crimes against humanity because of the deadly militant attack on a Nairobi shopping mall.

"The defense for Mr. William Samoei Ruto urgently requests... the trial chamber adjourn the trial proceedings in this case in order that Mr. Ruto can return to Kenya to deal with an ongoing and extremely serious matter of national security," the request from his lawyer said.

Kenyan troops launched an assault Sunday to end a terrifying siege by Somali militants holding hostages inside a shopping mall, with police warning the death toll of 68 could rise sharply.

Shortly after night fell with the siege well into its second day, teams backed by Israeli agents moved in to end the bloodbath at the upmarket Westgate complex.

Fierce gunfire could be heard coming from inside Nairobi's Westgate shopping mall Sunday, an AFP correspondent at the scene said, adding that two wounded Kenyan soldiers were evacuated from the complex and taken away by ambulance.

Kenyan security forces have been battling to end the siege in the upmarket mall for more than 20 hours, after fighters from Somalia's Al Qaida-linked Shebab group stormed the mall in an attack that killed at least 39 people and wounded 150 more.

British nationals are "undoubtedly" caught up in the deadly attack on a mall in Nairobi and diplomats are trying to obtain full details of any casualties, Foreign Secretary William Hague said on Saturday.

"There are undoubtedly British nationals caught up in this so we should be ready for that," Hague said in a televised statement following the attack, in which police say 30 people have been killed.

The U.S. State Department said Americans were reportedly among the injured in the attack Saturday at a shopping mall in Nairobi, condemning the "senseless act of violence."

"We have reports of American citizens injured in the attack, and the U.S. embassy is actively reaching out to provide assistance," spokeswoman Marie Harf said without elaborating, citing privacy concerns.

Kenya's president will not attend the U.N. General Assembly because his deputy will be away on trial at the International Criminal Court, State House said Friday, warning future cooperation with the ICC could be in jeopardy.

"Whereas very important multilateral and bilateral meetings had been planned for President (Uhuru) Kenyatta during the week, including a speech to the General Assembly, we very much regret that he cannot be out of the country at the same time as the Deputy President," a statement said.

Kenyan lawmakers on Thursday backed a motion to pull out of the International Criminal Court, an angry snub to The Hague-based tribunal ahead of next week's trial of Vice President William Ruto.

The symbolic vote offers a defiant message to the ICC, but does not impact upcoming trials of the east African nation's leadership, and parliament must now vote on a bill within 30 days to formalize steps for an actual withdrawal.