Gunmen killed 20 miners and injured seven others in an attack on a small private coal mine in southwest Pakistan late on Thursday, police said.

"A group of armed men attacked the Junaid Coal company mines in the Duki area in the wee hours using heavy weapons," police official Humayun Khan Nasir said.

What do we know about the attack?

The attack occurred east of the city of Quetta, the capital of the restive Balochistan province.

According to the police, the attackers stormed the Duki district mine and rounded up workers before opening fire.

They also fired rockets and grenades at the mines, Khan Nasir said.

Most of the victims hailed from Balochistan's Pashtun-speaking areas. Three of the dead and four of the wounded were Afghan.

No group has claimed responsibility for the attack yet.

Trouble in Balochistan province

Balochistan, Pakistan's southwestern province that borders Afghanistan and Iran, is the country's poorest and least populous province.

Rebel groups have waged a separatist insurgency there for decades, complaining that Islamabad and the richer Punjab province unfairly exploit their resources.

The Pakistani government has tried to use military force in an effort to put an end to the insurgency.

On Monday, two Chinese nationals were killed and several others injured in a massive blast near the Jinnah International Airport in Karachi, in an attack claimed by a group called the Baloch Liberation Army.

This week, the Interior Ministry alerted the country's four provinces to increase security measures, warning that separatist groups and the Pakistani Taliban might be planning attacks.

The warning comes as Pakistan hosts the summit of the Shanghai Cooperation Organization in the capital Islamabad next week. High-level Chinese representation is expected.

ss/rmt (AP, Reuters)