Despite Wednesday's diplomatic flurry, the bloodshed on the ground raged unabated as rebel fighters made a new drive into the capital.
Rebels fired several mortar rounds at the Syrian president's residence in Damascus early Wednesday, but failed to hit their mark, said Bassam al-Dada, an adviser to the commander of the Free Syrian Army, Col. Riad al-Assad.
"This was a very special operation that was planned for a while," al-Dada said by telephone.
Assad's current whereabouts are unknown, and the rebels' targeting of the palace, located in the Muhajireen district in the northwestern part of the city, was largely a symbolic strike on the Syrian leader's power.
Meanwhile, a judge was killed when a bomb exploded under his car, the second high-profile assassination of a top Assad loyalist in two days. The SANA state news agency said the judge, Abad Nadhwah, died instantly when the bomb was detonated remotely.
Rebels also fired mortars at a Palestinian refugee camp, activists said, apparently to try to break the resistance of a pro-government Palestinian faction. There are a half million Palestinian refugees in Syria.
When Syria's unrest began last year, the Palestinians struggled to stay on the sidelines. But in recent months, many Palestinians started supporting the uprising.
The Popular Front for the Liberation of Palestine-General Command, led by Ahmed Jibril, has remained loyal to Assad, however.
The group issued a statement Wednesday saying eight of its members had been "killed and mutilated" by the rebels. It was impossible to independently verify the allegation.
In London, British Foreign Secretary William Hague said talks with rebel military leaders would not involve advice on military tactics or support for their operations. Hague also insisted that Britain would not consider offering weapons to Assad's opponents.
Face-to-face meetings with military figures will take place outside Syria, Hague said. Diplomats from the U.S., Britain, France and Turkey are already scheduled to meet with Syrian opposition groups on Thursday in Doha, Qatar, though there has been no announcement that those talks will include contacts with rebel fighters.
Hague said U.K. diplomats will tell rebel commanders to respect the human rights of captured Assad loyalists, amid concern over abuses carried out by both sides.
"In all contacts, my officials will stress the importance of respecting human rights and international human rights norms, rejecting extremism and terrorism, and working toward peaceful political transition," Hague told lawmakers.
At the Zaatari camp, which houses about 40,000 of the estimated 236,000 people who have fled into Jordan from Syria, Cameron said he would press Obama to drive forward efforts to end the 19-month-old conflict.
Cameron plans to convene a meeting of Britain's National Security Council in London devoted entirely to Syria and to discuss how the U.K. can encourage Obama to pursue a more direct strategy.
"Right here in Jordan I am hearing appalling stories about what has happened inside Syria, so one of the first things I want to talk to Barack about is how we must do more to try and solve this crisis," Cameron said.
Talks with those who fled the violence has redoubled his "determination that now, with a newly-elected American president, we have got to do more to help this part of the world, to help Syria achieve transition," Cameron added.
Stringer reported from London. AP writers Matthew Lee and Bradley Klapper in Washington, Dale Gavlak in Zaatari, Jordan, Suzan Fraser in Ankara, Turkey, Christopher Torchia in Istanbul and Barbara Surk and Bassem Mroue in Beirut contributed to this report.