More than 300 people were killed in violence across Syria on Thursday, making it the deadliest day of a 16-month uprising against President Bashar Assad's regime, a monitoring group said.

The Syrian Observatory for Human Rights said in a Friday statement that a total of 302 people -- 98 soldiers, 139 civilians and 65 rebels -- were killed across the country, revising an earlier toll.

Russia's veto of a U.N. Security Council resolution to sanction Syria has exposed President Barack Obama's "lack of leadership" on the issue, White House hopeful Mitt Romney said Thursday.

Russia and China blocked U.N. action against Syria for the third time in nine months, triggering outrage by Western powers intent on heaping more pressure on Syrian President Bashar Assad to step down.

One more Syrian general crossed into Turkey Thursday, bringing to 21 the number of generals who have fled the unrest in neighboring Syria, a foreign ministry diplomat told Agence France Presse.

"One more Syrian general turned up at the Turkish border, becoming the third high-ranking officer crossing into Turkey over the last three days," said the diplomat, speaking on condition of anonymity.

Syrian rebels seized control of the Bab al-Hawa border post with Turkey on Friday after a fierce battle with Syrian troops, an Agence France Presse photographer at the scene reported.

The carcasses of burnt-out lorries were scattered across the scene of the battle. Some 150 armed rebel fighters were in control of the post, which lies opposite Turkey's Cilvegozu border crossing in the southern province of Hatay.

Syrian military forces have routed rebel fighters from the Midan district of Damascus, state television said Friday, but a monitoring group said fighting was still ongoing.

"Our brave army forces have completely cleaned the area of Midan in Damascus of the remaining mercenary terrorists and have reestablished security," the broadcaster said.

The United Nations on Thursday sent its top military official to Syria to take charge of an observer mission as the U.N. Security Council wrangled over the future of the operation.

Because of the growing dangers in Syria, European nations on the council have proposed giving the unarmed observers a final 30-day mandate to let it be closed. A Security Council vote could be held Friday, diplomats said.

Prince Bandar bin Sultan, the former Saudi ambassador to the United States, has been named the country's new intelligence chief, the royal palace announced on Thursday.

Prince Bandar, who retains his job as secretary general of Saudi Arabia's National Security Council, replaces Mogren bin Abdel Aziz, who resigned.

Ramadan will begin on Friday in most Arab countries, religious authorities announced, but in Oman and Syria as well as for Iraq's Sunni Muslims and in non-Arab Iran it will start on Saturday.

Egypt, Jordan, Kuwait, Qatar, Saudi Arabia, Tunisia, the United Arab Emirates and Yemen, as well as Lebanon's Sunni Muslims, were among those to announce a Friday start.

The killing of three top regime officials has dealt President Bashar Assad a severe blow, but he has no intention to give up his power, experts said on Thursday.

Both the regime and the rebels believe they are winning as clashes engulf Damascus, for the fifth consecutive day, and a day after an attack struck the National Security headquarters, killing Assad's brother-in-law Assef Shawkat, defense minister Daoud Rajha and crisis cell chief Hassan Turkmani.

The Syrian regime is in its last days, the head of the main exiled opposition group said on Thursday, warning that a veto by Russia and China at the United Nations could have "disastrous" consequences.

"The regime is in its final days," Syrian National Council chief Abdel Basset Sayda said following talks in Rome with Italian Foreign Minister Giulio Terzi shortly after the veto at the U.N. Security Council in New York.