About 60 fighters ambushed an Afghan police convoy as it slowed to cross a river in the southern mountains, sparking a fierce gun battle that left 19 officers dead and five missing, officials said Tuesday. It was the deadliest blow to date to the newly constituted security force.

In later violence, two rockets exploded near the U.S. Embassy in the center of the Afghan capital Wednesday, wounding two people hours before Secretary of State Condoleezza Rice was due to arrive on an official visit.

The attacks underlined the challenges facing Afghanistan's U.S.-backed government as it struggles to strengthen a fragile democracy while dealing with a rebellion by insurgents that has killed about 1,400 people in the past six months.

In the attack late Monday on the convoy, suspected Taliban rebels hiding behind boulders opened fire with heavy machine guns and AK-47 assault rifles, said Yusuf Stanikzai, a spokesman for the Interior Ministry. Several officers were killed immediately, but others among the 150 in the convoy returned fire, he said. Fighting raged for hours into Tuesday before the rebels fled on foot and several motorbikes.

Among the 19 dead was the deputy police chief of Helmand province, Stanikzai said. Four police officers were wounded and the five missing were feared to be either kidnapped or dead, said Ghulam Muhiddin, Helmand's provincial administrator. Four police vehicles were destroyed after being riddled with bullets.

Security forces rushed 200 extra police officers to the area and were searching houses and mountain caves for the guerrillas, but none was caught or killed, Muhiddin said. He said the rebels were believed to have fled across the nearby border into Pakistan. Afghan officials say that many insurgents base themselves on the Pakistani side of the rugged, largely unguarded frontier, which they sneak across into Afghanistan to launch attacks.

Another Interior Ministry official, Dad Mohammed Rasa, said the attack was "the deadliest ever on the police," a force that now numbers some 55,000 and was set up soon after U.S.-led troops ousted the Taliban from power in late 2001 for harboring Osama bin Laden.

Violence also continued in other areas of the country. A U.S. soldier was wounded when fighters opened fire Tuesday on his vehicle near Kandahar, a former Taliban stronghold, said Lt. Col. Jerry O'Hara, a U.S. military spokesman. Troops returned fire and the rebels fled.

In neighboring Zabol province, U.S.-led coalition and Afghan forces killed two Chechens and a Pakistani who were fighting alongside Taliban rebels, local government spokesman Ali Khail said. He said documents found on the bodies of the three identified their nationalities.