And officials have warned those planning to travel to Pakistan to postpone their trips until more information becomes available.

British nationals already there should stay indoors and listen to the news, particularly the BBC World Service, the FO said.

Robin Cook said he was "worried" by the situation

The statement was released after Pakistani soldiers moved against Prime Minister Nawaz Sharif's decision to sack the army chief.

Foreign Secretary Robin Cook said Britain would "strongly condemn" any unconstitutional actions by the army in Pakistan.

A Foreign Office spokesman said British officials in Islamabad and Karachi were monitoring the situation and would be working to provide "consular assistance" to British nationals in the country.

The country's main airports are closed and a curfew is in place.

There is no early indication of how many Britons might be in Pakistan, but the spokesman said many people there had dual British-Pakistani nationality.

"British nationals and their families in Pakistan should continue to adopt a low profile and keep in touch with consular wardens and the British High Commission," the spokesman said.

'Coup no surprise'

There was no immediate explanation for the army chief's sacking, but in recent weeks there have been reports of a rift between the army and the civilian government.

Prime Minister Nawaz Sharif is under house arrest

Tariq Azim, Secretary General of the UK Pakistan Cultural Foundation, told BBC News Online that the coup would not be cause for alarm among those with relatives in Pakistan.

He said: "The news will not come as much of a surprise to Pakistanis living here.

"The army has been a back seat driver in Pakistan for many years - it has always been involved in political affairs.

"But it is a very disciplined army. I don't believe the situation is in danger of getting out of control."

'Calm' in Islamabad

Peter Elwood, director of the British Council in Pakistan, called from the body's Islamabad office to tell officials in London that staff at their five centres were safe.

"He called us at around 3.45pm and told us that, although the television stations are down, it is peaceful and calm in Islamabad itself," spokesman Nigel Semmens said.

In all the Council has eight staff, 12 teachers and 19 consultants from the UK working from offices in Islamabad, Gilgit, Karachi, Lahore and Pashwar alongside 180 local staff.

A British Airways spokeswoman confirmed that they had staff based at Islamabad Airport and said every effort was being made to check that they were safe.

The Foreign Office has set up a telephone hotline for people planning to travel to Pakistan or for those inquiring about British nationals there.