About 60 militants ambushed a police convoy as it slowed to cross a river in southern Afghan mountains, sparking a fierce gunbattle that left 19 officers dead in the deadliest blow yet for the fledgling security force, officials said Tuesday.

In later violence, two rockets exploded near the U.S. Embassy (search) in the center of the Afghan capital Wednesday, wounding two people hours hours before Secretary of State Condoleezza Rice (search) was due to arrive on an official visit. Rice's visit on Wednesday is her second trip to Afghanistan as secretary of state.

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

In the attack late Monday on the police convoy, suspected Taliban rebels hiding behind rocks surrounded the vehicles as they slowed on a dirt road to cross the river in Helmand province, then opened fire with heavy machine-guns and AK-47 assault rifles (search), Interior Ministry spokesman Yusuf Stanikzai said.

Several officers were killed immediately, but there were 150 police in the convoy and the survivors returned fire, he said. Fighting raged for hours into Tuesday before the militants fled on foot and several motorbikes.

Among the 19 dead was Helmand's deputy police chief, Stanikzai said. Four police officers were wounded and five were missing and feared either kidnapped or dead, said Ghulam Muhiddin, the Helmand 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, but none of the militants were caught or killed, Muhiddin said.

"This was a devastating attack on the police," he said. "We are doing our best to track these militants down."

But he said the rebels were believed to have fled across the nearby Pakistani border. Many insurgents are believed to base themselves on the Pakistani side of the rugged, largely unguarded frontier, where they sneak into Afghanistan to launch attacks.

Another Interior Ministry official, Dad Mohammed Rasa (search), 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 forces ousted the Taliban from power for harboring Usama bin Laden (search) in 2001.

Violence also continued in other areas.

A U.S. soldier was wounded when militants opened fire Tuesday on his vehicle near Kandahar city, a former Taliban stronghold, U.S. military spokesman Lt. Col. Jerry O'Hara said. Troops returned fire and the rebels fled.

Militants fired two rockets into Kandahar before dawn Tuesday, but they exploded on an empty plot of land in the center of the city and hurt nobody. Three other rockets were found and defused on a nearby hillside, officials said.

In neighboring Zabul province, U.S.-led coalition and Afghan forces killed two Chechans 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. Afghan officials have warned that foreign militants linked to the Taliban and Al Qaeda have entered Afghanistan to fight.