Updated

Five Polish soldiers were killed Wednesday when a roadside bomb exploded in eastern Afghanistan, the alliance and a Polish official said.

It was the deadliest single attack for the Polish military in Afghanistan, spokesman Jacek Sonta said in Warsaw. He said the soldiers were in a convoy headed to Rawza, in eastern Ghazni province, when it struck the bomb.

The Taliban immediately claimed responsibility for the attack in a text message to journalists. Taliban spokesman Zabiullah Mujahid said "a Polish tank" was blown up and all its occupants were killed.

Mohamad Ali Ahmadi, deputy governor of Ghazni province, said that Polish soldiers were attending a morning meeting in the Rawza district of Ghazni city, about 77 miles southwest of the capital Kabul. One of their vehicles was destroyed by a roadside bomb, he said.

Poland is in the process of withdrawing some 100 out of the 2,600 troops it has in Afghanistan.

So far this year, 532 NATO service members have been killed in Afghanistan. The attack on the Polish convoy brings the number of NATO troops killed in December to 16.

Also in the east, Afghan police said they shot dead on Wednesday a would-be suicide bomber before he was able to attack a police station.

Youqib Khan, deputy police chief in Khost province, said policemen identified the would-be bomber in front of a bank next to the police station in Khost city and killed him before he could detonate his explosives vest. A search operation was under way because police fear the attacker, who was wearing a vest packed with explosives, might not have been working alone, he said.

Meanwhile, an Afghan military spokesman said the country's armed forces now number 180,000 troops, a significant step toward having enough troops to replace departing coalition forces.

Defense Ministry spokesman Mohammad Zahir Azimi said the troop numbers increased by more than 40,000 in 2011. This brings the military closer to the goal of having 195,000 Afghan troops by next October.

"Once we achieve that, then we will start discussing an agreement with international community to expand to 240,000 soldiers," Azimi said. "And once we have reached that, the Afghan National Army will be able to take control of internal and external security in all of Afghanistan."

The expansion of the army and police is a critical element in NATO's exit strategy from Afghanistan.

Coalition forces, which started their drawdown this year, are already handing over responsibility for security to the Afghan army and police in selected regions. The process will run through 2014 when international forces are to end their combat role.

But Azimi noted that troops are continuing to leave the Afghan military and that the attrition rate remains high, at about two percent per month. This makes it necessary to recruit and train large numbers of men just to keep up the army's strength levels.

Attrition includes all service members leaving the security forces, including those who have completed their terms of service or left due to medical or other reasons, losses in combat and desertions.

Critics have said some of the men deserting the security forces -- often with their weapons -- are defecting to the Taliban and providing the insurgents with trained new fighters.

Azimi attributed the losses mainly to the problems the men face when they are based far from their homes and families in regions in the east and south where the insurgency has been the most active.