Militants killed 17 road workers in Afghanistan's lawless south Tuesday, part of a spike in violence that has left 40 people dead in two days.

Sixteen other construction workers were wounded in the attack in Zabul's Shinkay district, said Interior Ministry spokesman Zemeri Bashary. Afghan and international security forces responding to the ambush killed seven militants and wounded 12, he said.

Building roads is a key part of development in Afghanistan, but many projects are in remote, insurgency-plagued areas. Militants have targeted work crews in roadside bomb attacks, ambushes and kidnappings. In January, militants in eastern Nuristan province beheaded four road construction workers.

The 40-nation military alliance in Afghanistan has stepped up efforts to contain the growing insurgency. The U.S. now has some 32,000 troops in the country, the most since the 2001 American-led invasion. Last year was the deadliest since the invasion, with more than 8,000 people killed, mostly militants, the U.N. says.

A Polish soldier died and one was wounded when a NATO patrol hit a roadside bomb in Ghazni province, next to Zabul, Poland's Defense Ministry said.

In southern Uruzgan province, militants attacked a police convoy Monday, and the ensuing clash left 13 insurgents dead and five wounded, said Bashary.

In the western province of Herat, Taliban militants attacked a checkpoint Monday in Shindand district, killing two police officers and wounding another, said Rauf Ahmadi, spokesman for the western region police.

Most Afghan police are poorly trained and equipped, leaving them vulnerable. Authorities say that more than 900 police were killed in insurgent attacks last year.