U.S. troops have tightened their grip around the Iraqi city of Najaf, where radical Shi'ite cleric Moqtada al-Sadr is believed to have taken refuge.

The deputy head of U.S. military operations in Iraq, General Mark Kimmitt, says U.S. troops will go after the cleric at the necessary time to end the violence started by his militia earlier this month.

Reuters news service reports the wanted cleric has appointed an envoy and asked him to convey peace proposals to the Americans.

Meanwhile, Iraqi mediators say they have secured an extension of a shaky cease-fire in the Sunni Muslim city of Fallujah, west of Baghdad. The mediators said the new 48-hour extension went into effect at 9:00 a.m. local time (0500 UTC) Wednesday.

Fallujah has been under a siege by U.S. Marines for about 10 days.

Tuesday, a U.S. official in Baghdad, Dan Senor, said alleged al-Qaida operative Abu Musab al-Zarqawi is believed to be in or near the city.

Also Wednesday, the U.S. military released a statement saying four Marines were killed in action during the past two days in areas west of Baghdad. The statement did not provide details.

At the United Nations, Secretary-General Kofi Annan says continuing violence in Iraq will prevent him from sending a large U.N. team back to the country "for the foreseeable future." He says violence has already hindered a small U.N. team working with Iraqis on forming a new government.