Two Chinese nationals were shot at and injured on Tuesday in Pakistan's commercial hub of Karachi, police and hospital officials said, after a string of attacks that spurred Beijing to demand tougher security for its citizens.

Faizan Ali, a senior superintendent of police, said two Chinese nationals had been shot, but gave no further details.

A spokesperson for Liaquat National Hospital in the southern port city said it was treating the two, one of whom was in serious condition.

It was not immediately clear who was responsible or if militant violence was behind the attack, one of several that have prompted Beijing to urge Pakistan to roll out more stringent security measures for its citizens.

In October, a bombing near Karachi's international airport killed two Chinese engineers, in an attack claimed by separatist militant group the Baloch Liberation Army (BLA).

A decades-long insurgency in the southwestern region by separatists demanding a share in regional resources has led to frequent attacks against the government, army and Chinese interests.