The United States is determined to increase pressure on Syria's President Bashar Assad to make him stand down, the U.S. envoy to the United Nations said Tuesday.

"The situation in Syria remains dire, especially for the millions who continue to endure daily attacks and are in urgent need of humanitarian assistance," ambassador Susan Rice told reporters after a Security Council briefing by U.N.-Arab League envoy Kofi Annan.

Amnesty International urged Bahrain on Tuesday to "immediately" release prominent rights activist Nabeel Rajab who was arrested over tweets deemed insulting to the government.

"Nabeel Rajab is a prisoner of conscience and he must be immediately and unconditionally released," said Philip Luther, head of the Middle East and North Africa program at the London-based watchdog.

Nearly 12,000 people, the majority of them civilians, have been killed in Syria since the outbreak in March 2011 of a revolt against the regime of Bashar al-Assad, a watchdog said Tuesday.

Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, said that of those killed, some 800 had died since a U.N.-backed ceasefire went into effect on April 12.

U.N.-Arab League envoy Kofi Annan said on Tuesday his peace plan could be the last chance to avoid civil war in Syria, where a truce has failed to end 14 months of bloodshed that monitors say has killed nearly 12,000 people.

Annan told the U.N. Security Council that the priority in Syria is "to stop the killing," and expressed concern that torture, mass arrests and other human rights violations are intensifying.

Former Libyan rebels angry over unpaid stipends opened fire on the headquarters of the interim government on Tuesday after surrounding the building.

There were no immediate reports of casualties.

A Bahraini court on Tuesday adjourned for two weeks an appeals hearing for 13 dissidents, including Abdulhadi al-Khawaja, a prominent activist on a hunger strike, a judicial source said.

Eleven of the defendants appeared in court except for Khawaja, who is at a military hospital as he has refused food since February 8, and Mirza al-Mahroos, who was also admitted to hospital, the source told Agence France Presse.

A suspected member of al-Qaida was killed when a bomb he was trying to plant at a checkpoint run by a local militia in Yemen's restive Abyan province exploded, a tribal source said on Tuesday.

The explosion took place late on Monday in Mudia, one of only two towns in Abyan -- along with Loder -- that has not yet fallen under the control of the extremists.

Iran said on Tuesday parliamentary elections in Syria held by the regime of President Bashar Assad was a step towards reform, but expressed regret the vote was shunned by opposition groups.

Foreign ministry spokesman Ramin Mehmanparast told reporters in his weekly briefing that "the vote is the second step the Syrian government is taking in the path of reform," after Assad's promise of launching reforms.

The International Committee of the Red Cross on Tuesday to step up its aid to Syria, where thousands have died in a 14-month bloody uprising against the regime.

The group, which has been working alongside the Syrian Arab Red Crescent to bring humanitarian relief to about 1.5 million people affected by the bloodshed, said it needed the extra $27 million to expand its efforts.

Interpol said Tuesday it had issued an international Red Notice for the arrest of Iraq's fugitive Vice President Tareq al-Hashemi "on suspicion of guiding and financing terrorist attacks".

"The Red Notice for al-Hashemi represents a regional and international alert to all of Interpol's 190 member countries to seek their help in locating and arresting him," the Lyon-based international police agency said.