Speaking during a visit to London, French Foreign Minister Jean-Yves Le Drian also condemned as "intolerable" comments by Chechen leader Ramzan Kadyrov after a knife-attack in Paris on Saturday.

Kadyrov had said that France bore "full responsibility" for the attack, which French police said was carried out by Chechnya-born French national Khamzan Azimov.

"We do not need lessons from a dictator who does not respect the... rule of law in his own country, and who also knows very well that there are thousands of Chechens fighting on the side of" Islamic State, Le Drian told AFP.

The attacker went on a stabbing spree in a busy district of central Paris on Saturday evening before police shot him dead, killing one person and seriously wounding four more.

Following talks with British counterpart Boris Johnson, Le Drian also spoke about the 2015 nuclear agreement between Iran and the 5+1 group (China, the US, France, Britain, Russia and Germany).

The agreement lifted international sanctions in exchange for Tehran promising to limit its sensitive nuclear activities and allow in international inspectors.

"Our position is one of determination and unity, of will to keep this agreement alive," Le Drian said.

"The US leaving an international agreement does not mean that the international agreement is null and void," said the French politician, on the eve of a Brussels meeting between the Iranian, French, British and German foreign ministers.

"We remain in the agreement and we want to keep it alive, as long as Iran respects it", he added, describing the agreement as "win-win".

"Our position is... united with the German position," continued the minister.

British counterpart Johnson said the "UK and France are determined to conserve the essence of the Iran nuclear deal.

"Tomorrow in Brussels, we are going to have a conversation about what we can do to help UK firms and help European firms have confidence that they can still do business.

"I'm not going to pretend to you now that it will be easy but we are determined to do that," he added, saying it was "vital that we continue to engage with the US".

Iranian Foreign Minister Mohammad Zarif on Monday in Moscow said he wanted "assurances" from the deal's signatories that they would continue to abide by its terms, especially on sanctions relief for Tehran.

He was in Moscow on Monday following talks in Beijing over the weekend.