Yemeni security forces opened fire on protesters in Sanaa calling for the ouster of President Ali Abdullah Saleh on Sunday, killing 20 people and wounding 500 others, medics said.

Medics in the capital reported 20 dead and 500 others wounded by live rounds, batons or suffering from breathing difficulties after inhaling tear gas.

Syrian President Bashar al-Assad on Sunday warned against "any foreign intervention that threatens to fragment states in the region and increase the risk of extremism in them," in talks with a Russian delegation on a mission to help end the government crackdown on anti-regime protests.

Assad also welcomed the "balanced and constructive Russian position toward the security and stability of Syria," the state-run SANA news agency reported.

Hamas on Sunday said it would not back a U.N. membership bid, and warned that no Palestinian leader had a mandate to sacrifice fundamental Palestinian rights.

Hamas prime minister Ismail Haniya, speaking in Gaza, said the group also continued to support the establishment of a Palestinian state on any part of "historical Palestine" but would not seek to disrupt the U.N. bid.

Opponents of President Bashar Assad met in Damascus on Sunday calling for the continuation of popular protests to overthrow the "tyrannical" regime in power in Syria.

"We need to end the tyrannical security regime. We must overthrow the tyranny and the security (agents). We welcome all those who have no blood on their hands," said Hassan Abdul Azim, a member of the opposition National Coordinating Committee for Democratic Change.

Israeli Prime Minister Benjamin Netanyahu said on Sunday that next week's bid by the Palestinians for U.N. membership had no chance of success and that they would ultimately seek renewed talks.

"Their attempt to be accepted as a member of the United Nations will fail," he said at the start of the weekly cabinet meeting. "That attempt will fail, since it has to pass through the Security Council."

The trial of members of a suspected al-Qaida cell charged with planning attacks against two U.S. military bases in Qatar opened in Riyadh on Saturday.

The cell, dismantled five years ago, comprised 41 members -- including 38 Saudis, a Qatari, an Afghan and a Yemeni -- and was "the most important al-Qaida" operation in Saudi Arabia, the attorney-general said.

Six students were wounded in clashes Saturday on the first day of the academic year between rival groups at Sanaa University, near the epicenter of ongoing protests, a hospital source said.

Two groups, divided between students who backed a resumption of classes and those who did not, came to blows, with stones and other projectiles also being used in the clashes, according to those involved and the medical official.

Syrian security forces killed two people on Saturday as they conducted searches in the northwestern province of Idlib, the Syrian Observatory for Human Rights said.

The victims, a man and a woman, were killed in the town of Khan Sheikhun, the British-based group told Agence France Presse by telephone.

Forces loyal to Moammar Gadhafi fired rockets and scored casualties in a fierce counter-attack in the fugitive ex-Libyan leader's stronghold of Bani Walid Saturday, an Agence France Presse correspondent said.

Ambulances rushed dead and wounded fighters as explosions and gunfire thundered in the air, after the attack on a position of interim government fighters several kilometers from the city center.

Turkey Saturday was parachuting in 22 tons of humanitarian aid to the Libyan city of Bani Walid, one of the last holdouts of fallen leader Moammar Gadhafi's supporters, the prime minister's office said.

Two military cargo planes left Ankara on Saturday to bring food to around 10,000 people in Bani Walid in need of urgent help, the office said in a statement on its website.