FALLUJAH, Iraq - Guerrillas and residents in Fallujah have "days, not weeks" to turn in heavy weapons, the top Marine commander in Iraq said today, warning that fighting could resume and that a U.S. push to take the city could be costly for both sides.

The stark warning by Lt. Gen. James Conway came two days after an agreement was reached in which city leaders called on insurgents to hand over their heavy weapons in return for a U.S. pledge to hold back on plans to storm the city and allow the return of families that fled the city.

Marines have said few weapons have been turned in and that most that were surrendered were old or didn't work.

In Baghdad, meanwhile, a foreigner was killed and his Iraqi translator was wounded by gunmen who opened fire on their bullet-proof vehicle today. The victim's nationality was not immediately known. The shooting took place in the northern neighborhood of Azimiyah, where gunmen have been active in past weeks.

A spokesman for British forces responsible for the southern Iraqi city of Basra has lowered the reported death toll to 50 from a series of near simultaneous suicide bombings that targeted Iraqi police stations there Wednesday.

The spokesman, Capt. Hisham Halawi, said a check with hospitals showed 50 dead, 20 of them children. Earlier reports had said 68 people were killed.

It was still too early to say who was behind the Basra attacks, Halawi said.

"We can't discount al-Qaida, we can't discount former regime loyalists. It is too early to start speculating," Halawi said in Kuwait.

Suicide attackers detonated five car bombs - all but one of them simultaneously - against police buildings in Iraq's second largest city, striking rush-hour crowds.

Vehicles were destroyed in the blasts, including two school buses carrying kindergartners and girls aged 10 to 15 that were passing by.

Police discovered two car bombs before they were detonated and arrested three men, said Basra Gov. Wael Abdul-Latif

Basra is overwhelmingly Shiite, and the last major suicide attack also targeted Shiites: a series of suicide bombers who struck holy shrines in Karbala and Baghdad on March 2. At least 181 people were killed.

In Fallujah, Conway's warning that residents have "days, not weeks" to implement the accord reached this weekend or else fighting could resume followed a disappointing surrender of weapons on Wednesday.

He said only about a pickup truck load's worth of weapons were turned in.

"It was junk, things I wouldn't ask my Marines to begin to fire," Conway said. "We were not pleased at all with the turn-in we saw yesterday."

Conway warned that patience is wearing thin and questioned whether the civic leaders who negotiated with U.S. officials had much influence over the insurgents.

"We are somewhat questioning whether they represent the people of Fallujah because it is our estimate that the people of Fallujah have not responded well to the agreement that was made in this very room," Conway said.

Marines believe that insurgents may have been using the lull in fighting to regroup. The gunmen have "had time to perhaps establish defenses better, install more ambushes. ... I think it will be costly but the Marines understand that," Conway said.

U.S. Marines halted efforts to allow families who had fled amid fighting to return home to Fallujah, a step that was to be taken alongside disarmament. About 10 families made it back into the city in the morning before Marines announced to some 600 Iraqis waiting at the checkpoint that no more would be allowed to enter.

The failure of Iraqi security forces to fight is significant because Washington's exit strategy depends on moving U.S. troops out of cities and turning over responsibility for security to Iraqi forces.