President Bush tried to keep an Iraq ally on board Saturday, receiving assurances that Bulgaria's (search) troop commitment in the country remains strong despite threats by insurgents to kill two Bulgarians held hostages.

The phone call between Bush and Bulgarian President Georgi Parvanov (search) took place as the Philippines announced it would withdraw its 51-member humanitarian contingent from Iraq as scheduled after its mandate ends Aug. 20.

Minutes after the announcement, the Al-Jazeera (search) television station aired a tape of a Filipino worker reportedly seized by guerrillas in Iraq appealing to his country's president to withdraw the troops from Iraq.

The Philippines still could decide to renew its troops' mandate, and it was not clear if the withdrawal was acquiescence or defiance to the insurgents' demands. The Philippines was told to withdraw its troops within three days, a deadline expiring over the weekend.

Bush and Parvanov discussed the hostage situation, with Bush offering to assist but refusing to negotiate with terrorists, the White House said. Parvanov pledged Bulgaria's strong commitment to Iraq.

A group loyal to insurgency leader Abu Musab al-Zarqawi threatened to kill the two Bulgarians if the U.S. military did not release all Iraqi detainees within 24 hours.

Bulgaria has a 480-member infantry unit in Iraq that is under Polish command in the city of Karbala. Its main duties are patrolling the center of the city and guarding public buildings.

Five Bulgarian soldiers died in a suicide attack against their base in December. In April, a sixth Bulgarian was killed in a skirmish with insurgents.

Bush was spending the weekend at the White House after three recent trips to states that are important to his re-election chances. On Saturday morning, Bush rode his mountain bike at a Secret Service training facility in suburban Maryland.