The United States on Thursday urged Tunisian leaders to come together to resolve the tensions wracking the country, warning "there's no place for violence in Tunisia's democracy."

Washington has condemned the assassination of leftist opposition leader Chokri Belaid, who was gunned down Wednesday outside his home in Tunis, sparking clashes between police and hundreds of protesters.

President Barack Obama said Thursday that the beliefs of U.S. gun owners should be respected, but warned measures must be taken to ensure hundreds or thousands more children are not shot and killed.

Obama, speaking to a conference of House of Representatives Democrats, pledged to keep campaigning for a package of measures designed to stem gun violence which were drawn up after the Newtown school massacre last year.

The U.S. military is not a 911 service ready to rush to every emergency around the world, Pentagon chief Leon Panetta told lawmakers Thursday, defending the response to an attack on a mission in Libya.

He also urged a Senate committee to help remove the threat of deep automatic budget cuts set to hit the Defense Department from March 1, calling them one of the greatest risks to America's national security.

WikiLeaks founder Julian Assange risks turning from a popular hero into an L. Ron Hubbard figure, tolerating only "blinkered, cultish devotion", said one his former backers Jemima Khan.

Claiming Assange had alienated his supporters, Khan, associate editor of the New Statesman, wrote for the weekly British magazine that Assange's anti-secrecy organization was now "guilty of the same obfuscation and misinformation as those it sought to expose".

Iranian supreme leader Ayatollah Ali Khamenei rejected Thursday a US offer to negotiate one-on-one on Tehran's disputed nuclear program so long as Washington continues to impose sanctions on Iran.

"You (Americans) want to negotiate when you are pointing the gun at Iran. The Iranian nation will not be intimidated by such actions," Khamenei told air force commanders, according to excerpts of his speech posted on his website.

The U.S. military will maintain only one aircraft carrier in the Gulf instead of two due to potential deep budget cuts that could kick in next month, Pentagon officials said Wednesday.

For several years, the United States has kept two carriers deployed in the Gulf due to tensions with Iran, but uncertainty surrounding the Pentagon's budget forced the decision, officials said.

The hard-nosed architect of the U.S. drone war against al-Qaida, John Brennan, will face tough questions about secret assassinations Thursday from senators weighing his nomination to lead the CIA.

The confirmation hearing before the Senate Intelligence Committee promises to focus a rare public spotlight on President Barack Obama's covert campaign to hunt down al-Qaida suspects worldwide in drone bombing raids.

The United States on Wednesday denounced the killing of prominent Tunisian opposition figure Chokri Belaid, describing it as an "outrageous and cowardly act."

"The United States strongly condemns this morning's assassination of Chokri Belaid, a Tunisian political party leader and a prominent opposition figure under the (Zine El Abidine) Ben Ali regime," the embassy in Tunis said.

Germany's cabinet on Wednesday proposed September 22 as the date for national elections in Europe's top economy, as a new opinion poll put Chancellor Angela Merkel's party 16 points ahead of her nearest rival.

The cabinet proposed the date after the main political parties and the majority of Germany's 16 states signed up to September 22.

The United States has sent an FBI team to join the Turkish investigation into a deadly suicide bombing on the U.S. embassy in Ankara last week, the mission said Wednesday.

Friday's bombing, the second attack on a U.S. diplomatic mission in five months, was claimed by a radical leftwing group the Revolutionary People's Liberation Front (DHKP-C).