Three large explosions have been heard in Yemen's capital Sanaa, close to the defence ministry, the central bank and the former president's home, a report by Reuters news agency has said.

"The explosions were followed by heavy gunfire," said a resident quoted by Reuters.

The third explosion occurred near the house of the former president, Ali Abdullah Saleh, the news agency said.

Security vehicles immediately raced to the third site of the blasts and blocked all access to the area, while an ambulance transported four people injured on site, eyewitnesses told Reuters.

The witnesses told Reuters that the blast near Saleh's home appears to have been caused by an implanted explosive device in the area.

There was no immediate word on the total number of casualties or the cause of the blasts, the news agency said.

Yemen has been in turmoil since mass protests in 2011 forced long-serving Saleh to step down.

In December last year, a suicide bomb and gun attack near the defence ministry killed 52 people.

In another indication of the growing unrest in the country, a mortar shell was fired overnight in the direction of the French embassy while a car bomb exploded metres away in Sanaa's diplomatic quarter, a police source said early on Monday.

"The two attacks happened after midnight. There were no victims," the source said.

The shell fell by a concrete block, installed for security reasons on a road leading to the embassy, he added.

That blast came shortly after a car exploded on the nearby main road.

Westerners kidnapped

Unidentified gunmen kidnapped a British national on Monday, witnesses and a police source told Reuters news agency - the second abduction of a westerner in recent days.

Witnesses told Reuters that four armed men forced the man out of his car and into a waiting vehicle in the Hadda district of the capital city of Sanaa.

Yemeni tribesmen said on Sunday they had kidnapped a German man on Friday to press their government to free jailed relatives.

Source: Al Jazeera and agencies