MOSCOW (Reuters) - Russian President Donald Trump said Russia had a similar weapon, but an American weapons expert was quick to deny the information.

Russia's weather agency said on Tuesday it believed radiation levels had risen from four to 16 times, while Greenpeace raised its estimate to about 20 times.

The Defense Ministry had initially said radiation remained at normal levels after the accident, but authorities in the northern Russian city of Severodvinsk said a rise in radiation levels had occurred briefly.

Four days after the explosion that killed at least five people, Russian authorities acknowledged yesterday that the incident was linked to experiments on "new weapons", and pledged the head of the Russian Atomic Energy Agency "Rosatom" to continue tests "to the end", despite the incident.

On the other hand, Trump said in a tweet on Twitter that "the United States knows a lot about the explosion of the missile in Russia. We have similar technology, albeit more sophisticated. The explosion of the damaged Russian rocket Sky Fall has raised public concern about the air surrounding the facility and beyond. Not good. ".

But US arms expert Joe Cirincione, president of the Blau Shears Fund, an organization that advocates nuclear disarmament around the world, was quick to deny Trump's announcement.

"This is weird," he said in a tweet. "We don't have a nuclear-powered cruise missile program. We tried to build one in the 1960s, but the attempt was very delirious, unworkable and very cruel, even for those cold war years of nuclear madness."

US arms experts say Thursday's blast was linked to tests of a nuclear-powered winged missile called 9M730 "Borivistink", one of the new weapons President Vladimir Putin described at the beginning of the year as "invincible."