March 17 - An air strike by Pakistan on Monday that the Afghan government says killed hundreds of civilians has led to an escalation in the conflict between the South Asian neighbours.

Here is a look at what happened.

WHERE WAS THE ATTACK?

The Afghan Taliban government says the Pakistani air strike targeted a drug rehabilitation hospital in the capital Kabul, killing at least 408 people and injuring 265 more.

Pakistan, however, rejected the claim, saying the strike targeted military installations and terrorist support infrastructure, and was carefully undertaken to ensure there was no collateral damage.

It has not verified the casualty numbers.

WHY HAVE THE NEIGHBOURS BEEN FIGHTING?

Allies-turned-foes Pakistan and Afghanistan's worst fighting in years erupted last month, with Pakistani air strikes inside Afghanistan that Islamabad said targeted militant strongholds.

Afghanistan called the strikes a violation of its sovereignty that targeted civilians, and launched retaliatory operations.

Over the last three weeks, both countries have launched air and drone strikes against each other and also engaged in ground firing across their 2,600-km (1,600-mile) border, with each claiming to have inflicted heavy damage and killed hundreds of opposition troops, without providing evidence.

HOW IS MILITANCY STRAINING THE RELATIONSHIP?

Islamabad has long accused the Afghan Taliban government of providing a safe haven to militants executing attacks on Pakistan from its soil.

It says the leadership of militant group Tehreek-e-Taliban Pakistan, as the Pakistani Taliban are formally known, and many of its fighters are based in Afghanistan, which also hosts armed insurgents seeking independence for the southwestern province of Balochistan.

The Taliban, however, has denied the allegations an...