Hundreds of Syrian dissidents debated strategies to oust Bashar Assad's regime Saturday in Istanbul as U.S. Secretary of State Hillary Clinton called for a halt to the "brutal" repression of protests.

A simultaneous gathering scheduled to take place in Damascus was cancelled after at least 28 civilians were killed, including 16 in the capital, in demonstrations on Friday, activists said.

Libyan leader Moammar Gadhafi on Friday said that the recognition by Western and regional powers of the rebel National Transitional Council was of no significance.

"Recognize the so-called National Transitional Council a million times: it means nothing to the Libyan people who will trample on your decisions," he said in a message to thousands of his supporters in Zliten, 150 kilometers east of Tripoli.

The U.S. government tightened sanctions against three Libya-controlled banks in Turkey, Tunisia and Lebanon on Tuesday, ratcheting up the pressure on leader Moammer Gadhafi's support.

The U.S. Treasury Department said it had targeted three foreign Libyan-owned banks, identified six additional companies subject to sanctions, and lifted sanctions against a former Libyan oil minister who defected from the Gadhafi regime last month.

A five-day U.N. summit with 48 leaders of the world's Least Developed Countries (LDCs) began in Istanbul Monday to discuss a new 10-year aid plan to help lift nations out of poverty.

"Investing in the LDCs can provide the stimulus that will help to propel and sustain global economic recovery and stability," said Ban Ki-moon, the secretary general of the United Nations, in his opening remarks.

Israel's deadly raid on an aid flotilla for Gaza was a tragedy that demands detailed investigation and must not be repeated, Russian Prime Minister Vladimir Putin told Agence France Presse in an interview.

"What is especially tragic is that this act was carried out in neutral waters," the Russian prime minister said in a discussion late Monday in the Black Sea resort town of Sochi.