Explosions have hit Yemen's capital, Sana'a, as the worst violence in a seven-month uprising against President Ali Abdullah Saleh continued for a third day.

The clashes Tuesday between government security forces and protesters calling for Mr. Saleh to step down come a day after pro-Saleh troops shot dead at least 31 people in Sana'a.

The government forces have killed at least 57 people and wounded 650 more since Sunday, when loyalist forces opened fire on tens of thousands of pro-democracy protesters.

In the southern city of Taiz, four more people were killed and 40 others wounded in clashes Monday.

The bloody crackdown has triggered fierce gunbattles between pro-Saleh troops and forces loyal to General Ali Mohsen al-Ahmar, who defected to the opposition months ago. Sana'a has since been split between General Ahmar's breakaway troops and loyalist forces.

Diplomats and Yemeni politicians are attempting to revive a long-stalled transition plan under which Mr. Saleh would hand over power. U.N. envoy Jamal bin Omar and the secretary general of the six-nation Gulf Cooperation Council arrived in Sana'a Monday and were expected to join the talks.

The Yemeni government said Monday it expresses “sorrow and condemnation” for the acts of violence that have occurred in the capital since Sunday. Foreign Minister Abu Bakr al-Kurbi told the U.N. Human Rights Council in Geneva his government will investigate and hold accountable those responsible.

U.N. Secretary-General Ban Ki-moon Monday strongly condemned the “excessive use of force” by government security forces, and he called for restraint. The U.S. embassy in Sana'a said it regrets the bloodshed and hopes a transition agreement can be reached within a week.

Mr. Saleh met Saudi Arabia's King Abdullah Monday in Riyadh. The Yemeni leader remains in Saudi Arabia, where he has been recovering from injuries sustained in a June attack on his presidential compound.

Yemen's youth-led protest movement stepped up demonstrations last week, angry after Mr. Saleh instructed his deputy to negotiate the power-sharing deal. Many call the move the latest of the president's delaying tactics.

Mr. Saleh has agreed to the GCC proposal three times since April, but always has backed out before a deal could be signed.