Kabul, Afghanistan ? Fighters loyal to rival warlords clashed Saturday in western Afghanistan, sending tanks into the streets of a regional capital in the latest jolt to the country’s shaky security ahead of national elections. More than 20 fighters were killed by one estimate.

The U.S. military expressed concern about the violence but showed no sign of intervening. So far, U.S.-trained troops in Afghanistan’s national army were also staying out of the fight — a sign of the central government’s weakness in the face of local warlords.

The clashes pitched forces loyal to Herat Gov. Ismail Khan, one of the country’s most powerful warlords, against rivals in the north, east and south of the province.

In the fiercest clash, a commander from Shindand, about 370 miles west of the capital, Kabul, said his men seized a Soviet-built air base.

“By 4 a.m. we had captured the whole of the district, including the airport and the division,” Amanullah, an ethnic Pashtun commander who goes by one name, told The Associated Press.

Afghan forces have few aircraft, but the base is home to a militia division believed loyal to Khan, a Tajik.

Amanullah said his fighters, armed with machine-guns and rockets, had killed 14 of Khan’s men and captured another 20. Seven of his own men also died, he said.

One of Amanullah’s commanders, Abdul Karim, said three more of his fighters were wounded, and a director of the hospital in Herat said three patients had been admitted from Shindand with minor injuries: two civilians and a soldier.

An Associated Press Television News reporter in Herat saw tanks heading south toward Shindand. Knots of militiamen stood guard at major junctions in the city.

Abdul Wahed Tawakali, a spokesman for Khan, said there was “hand-to-hand fighting” near the base, but denied it had fallen.

He had no information on casualties.

The battles are the latest in a string of factional clashes across the north and west of the country.