U.S. forces in Afghanistan will celebrate the Fourth of July with barbecues, volleyball and music — but no fireworks. 

Security around the perimeter of Bagram air base, the dusty headquarters of the 7,000 U.S. troops in Afghanistan, will be tightened Thursday, following warnings that terrorists could target Americans on their Independence Day. 

Col. Roger King, the U.S. military spokesman, said guards would take extra caution when searching people and cars entering the base. 

The U.S. government says it has credible evidence that terrorist violence may occur on July 4. It hasn't named any specific potential target. 

The Bagram base north of the capital, Kabul, is heavily fortified and guarded; U.S. forces stationed here have been warned to take precautions. 

Soldiers at the base said they were proud to be representing the United States in Afghanistan on July 4. 

"I definitely have a huge sense of pride to be able to serve my country here after Sept. 11," Capt. Krista D'Andrea said Wednesday. "I never thought I'd be here, that's for sure. But I miss my family, I wish I could be with them." 

D'Andrea, who has a 15-month-old son in San Antonio, Texas, said she'd already received a July 4 card from her husband, who wrote that he's proud of her. 

Some soldiers said they would celebrate the holiday — normally marked by family picnics and fireworks displays — by re-enlisting. 

Other nations' forces at the base would be invited to join the celebrations. A volleyball tournament will be held in the Polish compound and soldiers from various countries have been asked to the Americans' barbecue. 

July 4 will also mark the start of the withdrawal of British commandos the Royal Marines from Bagram. Two companies — about 300 men — of the 1,700-strong Royal Marines contingent will leave Afghanistan on Thursday. 

The bulk of Britain's 4,000 troops currently in Afghanistan are being withdrawn, including the commandos and forces taking part in the international peacekeeping force in Kabul.