More US B-52 bombers have been out on training flights from RAF Fairford in Gloucestershire.
Two of the 14 long-range aircraft stationed at the Cotswold air base touched down on Saturday after flyng for just over three hours.
The giant planes were also involved in similar training exercises on Friday.
They are expected to be at the forefront of any initial attack against Iraq.
Security tightened
A Ministry of Defence spokesman said the flights were part of a programme of routine test and training runs.
He said: "They need to make essential air and mechanical check flights as part of the serviceability and maintenance requirements.
"In addition they are permitted to undertake routine training flights."
Security at the RAF base has been increased this weekend following a number of break-ins by peace protesters.
Two people have been charged with damaging B-52 support vehicles.
Stop and search
RAF Fairford has been the scene of several protest marches against a possible war with Iraq and a peace camp has been set up nearby.
Gloucestershire Police last week issued a "Section 60 Order" covering a one-mile radius around the base allowing them to stop and search anyone in the area.
RAF Fairford is a NATO-designated standby base and is shared with the US Air Force.
In 1999 a group of women set up a camp outside the base to protest against the NATO bombing of Kosovo.
It is one of only three forward USAF bases outside America.
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