U.S. President Barack Obama must seize the opportunity offered by the G20 summit to settle bitter differences over Syria with Russian counterpart Vladimir Putin, former Soviet leader Mikhail Gorbachev said Wednesday.

The G20 summit, which starts in Russia on Thursday, comes with many analysts describing U.S.-Russia relations as at their lowest ebb since Gorbachev helped end the Cold War two decades ago.

U.S. President Barack Obama Wednesday urged world support for punitive strikes against Syria for its alleged use of chemical weapons, while Damascus vowed retaliation and resistance even if a third world war erupts.

Obama, fresh from efforts in Washington to secure bipartisan support for military intervention, said in Stockholm that the world had set "a red line" for Syria and it could not now remain silent in the face of the regime's alleged use of chemical weapons.

Blocked by his own parliament from taking military action against Syria, British Prime Minister David Cameron on Wednesday urged the United States to act or risk further chemical weapons attacks by the regime.

"(U.S. President Barack Obama) set a very clear red line that if there was large-scale chemical weapons use something had to happen," Cameron told lawmakers in the House of Commons.

Russian President Vladimir Putin on Wednesday warned the U.S. Congress it would be approving an "aggression" against Syria if it allowed U.S.-led military strikes against the regime of President Bashar Assad.

"They would be allowing an aggression since everything that is outside the framework of the U.N. Security Council is an aggression, unless it is self-defense," Putin told members of his human rights council in the Kremlin, Russian news agencies reported.

Syria said Wednesday it had taken "every measure" to retaliate if hit by a feared U.S.-led military strike and would never give in, even if a third world war erupts.

In an exclusive interview with Agence France Presse, Deputy Foreign Minister Faisal Muqdad also insisted that Russia had not wavered in its support, despite comments by President Vladimir Putin suggesting a more conciliatory tone towards the West.

A jihadist group on Wednesday poured scorn on the Egyptian military's claims of victories in the restive Sinai peninsula where the army said it launched deadly air strikes on militants.

The fiercely anti-Israeli group al-Salafiya al-Jihadiya, in a statement posted on Islamist forums, condemned "the state media and the army as liars" who "celebrate ... fake victories in the Sinai."

Middle East peace talks have so far proved "futile," a senior Palestinian official said on Wednesday, calling for greater U.S. participation in talks.

"Until now there has been no progress," the Palestine Liberation Organization's Yasser Abed Rabbo told Voice of Palestine radio, after several weeks of meetings between Israeli and Palestinian negotiators.

A delegation from the African Union, which suspended Egypt after Islamist president Mohammed Morsi's ouster, on Wednesday urged all parties in the country to back the political transition.

"We hope that all parties meet together around the roadmap" announced by the new authorities after Morsi's July 3 ouster, Alfa Omar Konari, who headed the three-member delegation that arrived in Egypt last week, told reporters.

Five Saudis accused of belonging to al-Qaida and of plotting attacks went on trial Wednesday in a Yemeni court set up for terrorism cases, the official Saba news agency reported.

The five men face charges of plotting "in association with an armed group belonging to al-Qaida to carry out criminal acts against members of the armed and security forces in Yemen," Saba said.

Islamist rebels in Syria seized an army checkpoint leading into the Christian town of Maalula on Wednesday, a rights watchdog said, as fighting rages on the ground.

"Jihadists from the Al-Nusra Front and Islamist rebels attacked a regime checkpoint on Wednesday morning at the entrance to Maalula, killing eight soldiers," the Syrian Observatory for Human Rights reported.