A top U.S. military official visiting Iraq says foreign fighters are still infiltrating Iraq through the borders of Syria and Iran and that additional resources may be needed to stabilize the country.

The chairman of the Joint Chiefs of Staff, General Richard Myers, said "we know for a fact" that Syrian and Iranian insurgents are crossing the border into Iraq to destabilize the situation.

The general, who on Wednesday began a surprise visit to Iraq to assess the fighting between coalition troops and Shiite and Sunni Arab resistance, told a news conference that he believes many insurgents are angry about the success being made toward Iraqi self-rule.

?And I think it is that success which is driving the current situation, because there are those extremists that don't want that success,? General Myers said. ?They see this as a test of wills a test of resolve against those who believe that freedom and self-determination against those that prefer a regime like we saw previously in Afghanistan, or perhaps a regime like we saw previously in Iraq. I think it is their frustration that has generated a lot of the situation we have seen now as well.?

Some 2500 coalition troops are surrounding the holy city of Najaf, where the radical Shiite cleric Moqtada al-Sadr is said to be located. U.S. forces have vowed to kill or capture him for inciting violence against the coalition. An Iraqi judge has issued a warrant for his arrest in connection with the murder of a rival cleric last year.

An Iranian diplomat was shot to death in his car Thursday, shortly after an Iranian delegation began attempts to mediate between the cleric and U.S. coalition authorities. Iran's Foreign Ministry said the killing was most certainly linked to the visit.

Meanwhile, an Italian hostage was executed by his captors, marking the first killing of a hostage since a recent spate of kidnappings began targeting foreigners inside Iraq.

Three Japanese hostages were set free after seven days in captivity and threats that they would be burned alive if Japanese forces did not leave Iraq.