US military satellites secretly monitored Sri Lanka’s conflict zone through the latter stages of the war against the Tamil Tigers and American officials are examining images for evidence of war crimes, The Times has learnt.

The images are of a higher resolution than any that are available commercially and could bolster the case for an international war crimes inquiry when the UN Human Rights Council holds a session on Sri Lanka next week.

They were acquired by the National Geo-spatial Intelligence Agency (NGA), based in Bethesda, Maryland, which is part of the Department of Defence but provides services for other government agencies.

Marshall Hudson, a spokesman for the NGA, told The Times that the agency had been monitoring the conflict zone and had provided images to the State Department, some of which were released to the media in April.

“It’s a safe assumption that we didn’t release everything that we have,” he said. He declined to give further details.

Other US officials said that the Office of War Crimes Issues was investigating Sri Lanka and that satellite images were a crucial part of the investigation because of the lack of access on the ground.

Sri Lanka declared victory in its 26-year civil war on Tuesday after killing or capturing the last of the Tigers.

Britain, the EU and Ban Ki Moon, the UN Secretary-General, have called for an investigation into allegations that both sides committed war crimes repeatedly, including firing on civilians.

European Union states are struggling to raise more than 17 votes on the 47-member Human Rights Council, dominated by a bloc led by China and Russia that has frequently prevented inquiries into human rights.

The US, which was elected to the Council last week after ending its boycott of the body, does not become a voting member until next month but is expected to speak at the meeting and could share its evidence with undecided members, diplomats said.

If the UN fails to back a war crimes inquiry Washington could use the images and others from commercial sources as evidence in its investigation, according to human rights activists.

This is the latest example of how satellite technology is being used to monitor conflicts and hold governments to account for their actions.

Satellite imagery is valuable in the case of Sri Lanka because the Government has banned almost all independent aid workers and journalists from the front line, blocking examination of alleged war crime scenes.

The State Department has already used NGA satellite images to put pressure on the Sri Lankan Government.

It released two pictures to the media in April that it said showed 100,000 civilians crammed on to a beach in the conflict zone.

In the same month, the UN leaked satellite images from multiple sources that appeared to prove that the Sri Lankan air force had bombed civilians there despite establishing it as a no-fire zone for them to shelter in.

Sri Lanka admitted bombing the area but said that it was attacking Tiger artillery positions and that there were no civilians in the immediate area at the time. It accused the UN of spying.

Human Rights Watch has used satellite images of Sri Lanka from the American Association for the Advancement of Science, which has helped to expose rights abuses in Burma, Zimbabwe, Chad and the Darfur region of Sudan.

The resolution of the images does not exceed half a metre per pixel, and most do not allow night vision.

“We can do a little better than that,” Mr Hudson said. The NGA uses software to recognise and analyse differences between images that could indicate damages from bombs or heavy artillery.

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