United Nations peacekeepers have a responsibility to protect civilians where they are deployed even if that means using force, the US envoy to the world body said Friday during a visit to India, one of the biggest troop contributors.

Samantha Power said UN peacekeepers could not "simply stand by as atrocities are committed," as many had in the past, and that a failure to act had likely led to thousands of civilian deaths.

New Delhi has expressed open resistance to the move towards a more robust UN force, saying its understanding of peacekeeping does not include combat.

"We should also expect that peacekeepers in volatile situations use force when needed to defend themselves, to protect civilians or to otherwise carry out their mandate as authorised by the Security Council," said Power.

While these requirements already existed, she said, cases where peacekeepers fulfilled them remained "more the exception than the rule".

Power was speaking on a visit to India, which is currently the third-largest troop provider to the UN's 16 missions worldwide, with Indian peacekeepers in South Sudan, Western Sahara and the Democratic Republic of Congo.

She cited a report by the US internal oversight office in March last year that found in 507 attacks against civilians from 2010 to 2013, peacekeepers "virtually never used force to protect those coming under attack".

"Thousands of civilians likely lost their lives as a result," she said.

"This cannot go on, not for the institution of peacekeeping and certainly not for the people the blue helmets are entrusted with protecting," she added, describing the troops' trademark headgear.

Power also condemned recent instances of alleged abuse by UN peacekeepers including in the Central African Republic.

The UN mission in CAR has been hit by 13 cases of alleged sexual abuse by its peacekeepers, including nine that involve underage victims as young as 11.

The nationalities of the troops involved have not been revealed.

Power's message came after more than 50 countries pledged 40,000 troops for UN peacekeeping at a US-led summit.

US President Barack Obama told leaders at the September summit that peace operations were "experiencing unprecedented strains" and deployed in "more difficult and deadlier conflicts".

The pledges represented a major boost to UN peacekeeping, which relies on voluntary contributions.

Most are from poorer countries -- Bangladesh, Ethiopia, India, Pakistan and Rwanda are currently the top five troop-contributing nations to peace missions.

Power will head to Sri Lanka on Saturday to meet senior officials in the new government on the second leg of her visit to the region.