Militants attacked government offices and a police convoy Saturday, continuing a series of assaults that has left at least 41 people dead in the region over two days, government officials said.

About 250 Afghan forces fought more than 200 rebels in the area's fiercest fighting in months. At least 19 people were killed on in Afghanistan and Pakistan Saturday.

Afghan officials said U.S. forces joined the battle Friday and Saturday but a U.S. military spokesman said he could only confirm involvement in the first day of fighting.

The violence spread across the border as a roadside bomb exploded by an army vehicle Saturday in Pakistan in a northwestern tribal region near Afghanistan, killing three security personnel, an official said.

Nobody immediately claimed responsibility but security officials have blamed Al Qaeda-linked Islamic militants for previous attacks in the area, where Pakistan has sent thousands of troops to flush out insurgents.

The bloodshed Saturday underscored the challenge facing thousands of British and Canadian troops in coming months as they gradually relieve American forces in southern Afghanistan, a hotbed of insurgency and the drug trade.

Fighting began Friday with a mountain ambush of a police convoy that left 16 militants and six police dead and scores wounded, said Amir Mohammed Akhund, the province's deputy governor.

American and British war planes bombed suspected Taliban militants fleeing the fighting around midnight Friday, killing eight of them, said Khan Mohammed, a police chief in Helmand province.

Another group of militants fleeing the initial clash attacked a government office early Saturday, killing the government chief and wounding four police, Akhund said.

Later in the day, another group of militants attacked the main government office in a neighboring district, setting off a two-hour gunbattle that left one policeman and three suspected Taliban dead, he said.

The fighting prompted dozens of families to flee their villages across the violence-wracked area, Mohammed said.

Militants used a remote-controlled bomb to attack a police convoy in Kandahar, the main city in southern Afghanistan and a former Taliban stronghold, said Sher Mohammed, a police officer.

A woman and a child who were walking in the area were killed, and three other passers-by were wounded, he said.

Also in Kandahar, a Taliban commander, Abdul Samad, was killed by border forces as he tried to enter illegally from neighboring Pakistan, Kandahar Gov. Asadullah Khalid said. Ten other militants fled back across the frontier.

The violence comes after an unprecedented spate of suicide bombings that have added a new security threat in the country four years after the ouster of the Taliban.

Fighting last year left some 1,600 people dead, the highest death toll since 2001, as militants stepped up their campaign against the U.S.-backed central government.

Afghan authorities blame much of the violence on foreign militants. An Iraqi was caught this week trying to sneak into the country and after interrogating him officials said they believed a large group of Arab al-Qaida militants were heading to Afghanistan.

The latest fighting came days after an international donors' conference in London ended with $10.5 billion in new aid pledged for Afghanistan — much of it for improving security.