A claim by the French interior minister that one of the bombs sent as air cargo from Yemen was found 17 minutes before it was due to explode was today dismissed by security officials in several countries.

It is believed that the minister, Brice Hortefeux, was referring, during a television interview, to the bomb found disguised in a printer cartridge on a plane that landed in Britain last Friday.

The explosive device with the chemical compound PETN was one of two sent as air cargo and addressed to synagogues in Chicago by al-Qaida in the Arabian Peninsula.

Officials in Britain dismissed the French minister's claims. One told the Guardian that the device found at East Midlands airport was still being examined by scientists, and that investigators were not yet sure when and where the terrorists had intended to detonate it.

Scepticism about the French claim also came from security officials in Dubai and from the White House in Washington.

The question of when the bombs were set to go off is central to the investigation because it could indicate whether the terrorists wanted to blow up the planes in US airspace or take down the aircraft regardless of location.

"One of the packages was defused only 17 minutes before the moment that it was set to explode," said Hortefeux. He made no other statement about the Yemen bomb plot during the interview on the state-run channel France-2 which focused on other security-related matters. His office at the interior ministry refused to give more details.

"If this was a reference to the device found in the Federal Express [FedEx] site in Dubai, then it is not correct," the source in the United Arab Emirates said.

The White House press secretary, Robert Gibbs, said the question of when the bombs found in Britain and the UAE were to explode, was still being investigated and there was no information confirming that a device was defused with just 17 minutes to elapse. The device found in Dubai had been carried as cargo on passenger planes before its discovery following intelligence passed to the west by Saudi Arabia, without which it was feared the bombs would not have been detected.

Under security measures announced today in the wake of the cargo bomb plot, freight from Pakistan, Libya, Iran, Sudan and India passing through UK airports, will have further screening before being allowed to be sent on.

Philip Hammond, the transport secretary, said cargo carried on certain routes by airlines including British Airways, BMI and Emirates, would no longer be exempted from extra checks. The services affected by this week's rule change include BA flights from the Bangladeshi capital, Dhaka, BMI services from Tehran, and a slew of Emirates routes to the UK from Karachi in Pakistan, the Sudanese capital Khartoum and Mumbai.

"We are withdrawing some of the trans-shipment exemptions that allowed some freight from some countries to come through our airports and be shipped on without being screened again," said Hammond.

Speaking after a security meeting with aviation industry representatives, the secretary of state said that a government review of cargo safety measures would consider a "layered" regime whereby freight from some countries would be banned and there would be enhanced checks on deliveries from countries that carried some risk.

Freight from "safe" countries, such as from EU member states, would be afforded easier access into the UK. "We are going to look at categorising countries according to the level of risk," said Hammond.

Freight from Yemen and Somalia was banned by the home secretary, Theresa May, after the discoveries at East Midlands airport and in Dubai.