"Russia continues to accuse falsely Ukraine for preparing and making a dirty bomb -- that is absurd, because why should Ukraine use a dirty bomb on the territories they want to liberate?" Jens Stoltenberg said during a visit to a US aircraft carrier currently in the Mediterranean, USS George H.W. Bush.

Western leaders have rejected Moscow's claim that Ukraine is planning to set off a crude device that could spread nuclear, chemical or biological materials over a wide area.

They fear that the Kremlin, which has faced major setbacks in its invasion of Ukraine as NATO countries back Kyiv with weapons and funds, is preparing a "false flag" operation where it launches such am attack and blames it on Ukraine.

"The world would see through any attempt to use the allegation as a pretext for escalation," the United States, France and Britain said in a joint statement.

The UN Security Council on Tuesday was to discuss the "dirty bomb" claim.

"I would be careful speculating, but we've seen this before, we also saw it at the start of the war," Stoltenberg said in a video interview with AFPTV.

"A lot of false accusations against Ukraine were used in a way to 'excuse' the invasion that happened later.... We have seen what has happened before and that makes it necessary to follow closely what Russia now does."

Stoltenberg added: "They need to understand that we will not accept a false pretext so that Russia further escalates the war in Ukraine."

- 'Powerful message' -

During his visit to the US warship, Stoltenberg addressed more than a hundred servicemen and women on board in the flight hangar, in front of a stage with a giant US flag and a line of national flags of NATO's 30 member countries.

The hangar held a couple of F-18 multi-role fighter jets and a portrait of the aircraft carrier's namesake, former US president George H.W. Bush.

The warship was leading a carrier strike group, a flotilla of US Navy vessels that includes a cruiser and four destroyers.

Stoltenberg told the personnel in the aircraft carrier that the strike group "sends a powerful message" that NATO allies are in a state of "increased vigilance from the Baltic to the Mediterranean -- and the Black Sea" which is off southern Ukraine.

While NATO is not directly involved in Ukraine's struggle against the Russian invasion, it has repeatedly said it will vigorously defend every centimetre of territory in NATO member states that neighbour Ukraine should they be attacked.

US President Joe Biden has warned Moscow that it using a nuclear weapon in its war in Ukraine -- including a lower-yield so-called tactical nuke -- would be "an incredibly serious mistake".