Shouting "We want revenge now!" hundreds of Hamas supporters marched in the West Bank and Gaza Strip Friday and vowed new suicide bombings after Israel killed the group's two top fugitives.

In two towns, protesters clashed with Israeli troops.Hamas, an Islamic militant group, promised to avenge the deaths of brothers Imad and Adel Awadallah. Bracing for attacks, Israel sealed off the West Bank and Gaza Strip and security forces were placed on high alert.

The new tension came as U.S. Mideast envoy Dennis Ross worked with the Israeli and Palestinian leaders on a deal for a West Bank troop withdrawal.

A new outbreak of violence would make it exceedingly difficult for Ross to negotiate an end to the 17-month stalemate.

On the third day of his mission, Ross met today with Palestinian security chiefs to discuss the possible release of Palestinian prisoners in Israeli jails. Later in the day, Ross held talks with Israeli Prime Minister Benjamin Netanyahu.

The killings of the Hamas fugitives on Thursday triggered protest marches and clashes.