But the RAF pilots were frustrated yet again as they were unable to deliver their weapons, said Group Captain Ian Travers Smith.

On Saturday night they were forced to return to base when a technical problem onboard a support aircraft saw their mission abandoned.

In the latest mission the Harrier GR7s were to strike at mobile Serb units, in an operation believed to among the largest yet launched by Nato.

But their targets, which had been identified by other aircraft, had moved by the time they reached the operational area.

A similar mission undertaken last week was halted by the poor weather which has seen the crews, operating from the Gioia del Colle base in southern Italy, complete only two successful raids.

The Harriers have had only two successful missions so far

Group Captain Travers Smith admitted the setbacks were causing increasing frustration for the pilots.

"This is yet another form of frustration. Now the weather has cleared they have taken off but there is nothing for them to hit," he said.

"This was the first of the daytime operations for the GR7s that have managed to get airborne (without weather problems).

"Their prime purpose today would have been to target mobile targets that would have been identified by other means.

"During the period the GR7s were airborne, no such opportunities presented themselves so the Harriers have returned with their bombs."

Daylight sorties are more dangerous for the pilots and there was an air of tension as ground crew and journalists watched the pilots launch at about 1200 GMT.

Number 1 Squadron's commanding officer, who cannot be named for security reasons, was the first to land back at the base.

Harriers are now flying daylight missions

"Because this is a new phase, tensions are there because the pilots are facing something they have not done yet," said the group captain.

"This is actually a new experience in this operation for them. They will be tense about doing anything new."

He added that the pilots would undergo an intensive debriefing - potentially lasting up to four hours - to ensure that all the lessons that could be learned from the sorties were taken on board.

At Sunday's Ministry of Defence briefing Air Marshal Sir John Day said improving weather in the Balkans would see the strikes intensified.