Nato is reviewing the conduct of its military campaign in Libya after France admitted arming rebel fighters in apparent defiance of the UN mandate.

The revelation surprised officials in Nato's headquarters in Brussels and raised awkward questions about whether the French had broken international law – UN resolution 1973 specifically allows Nato nations to protect civilians in Libya, but appears to stop short of permitting the provision of weapons.

Nato has consistently said it would not arm rebel commanders, saying it was beyond its remit. But military chiefs in Paris confirmed that French planes had dropped consignments of machine guns, rocket-propelled grenades, and anti-tank missiles to rebels in the western Nafusa mountains. A report in Le Figaro newspaper said the French parachuted "large amounts" of munitions to help the rebel push on Tripoli earlier this month.

This was confirmed by French armed forces spokesman Thierry Burkhard. He said they had initially provided humanitarian aid of water, food and medical supplies to civilians in the region under siege from regime forces. "There were humanitarian drops because the humanitarian situation was worsening, and, at one point, it seemed the security situation was threatening civilians who could not defend themselves," Burkhard told Reuters.

"France therefore also sent equipment allowing them to defend themselves, comprising light weapons and munitions." They were "self-defence" assets, he said. It appears France did not inform any Nato allies of the weapons drop, or Nato headquarters, where officials were today seeking clarification from Paris.

Nato was also trying to establish what legal basis France had for taking this apparently unilateral action. Officials expressed surprise at what had happened and insisted its military approach had not changed. "Nato knows what its mission is, and that the mandate allows certain things," said a source.

France's admission highlights tension within Nato over conduct of the campaign, and will raise fresh questions over whether the coalition should be doing more to hasten Muammar Gaddafi's downfall. Some countries are privately likely to welcome any sign of a more proactive effort to end Gaddafi's 41-year rule. Italy's foreign minister, Franco Frattini, has previously claimed that the UN resolution should not prohibit providing weapons to the rebels, saying this could be "morally justified".

In a further sign of growing frustration, the Dutch defence minister, Hans Hillen, criticised the Nato campaign , saying those allies who had thought bombing would force Gaddafi to step down were "naive". He also insisted that Nato's mission should be confined to its mandate to protect civilians.

Last night Barack Obama defended the Libya mission, saying he had operated within legal guidelines in authorising US involvement. "We have engaged in a limited operation to help a lot of people against one of the worst tyrants in the world," he said, referring to Gaddafi.

The Ministry of Defence said British forces had not supplied any weapons, though the Foreign Office acknowledged the UN resolution could be interpreted in different ways. "Our position is clear," a spokesman said. "There is an arms embargo in Libya. At the same time, UN resolution 1973 allows all necessary measures to protect civilians and civilian populations from the threat of attack. We think that the UN resolution allows, in certain limited circumstances, defensive weapons to be provided. But the UK is not engaged in that. Other countries will interpret the resolution in their own way."