Russian President Vladimir Putin said on Tuesday that last year's blasts on the Nord Stream gas pipelines had been carried out on a "state level", dismissing the idea an autonomous pro-Ukraine group was responsible as "complete nonsense".
The Nord Stream 1 and 2 pipelines connecting Russia and Germany under the Baltic Sea were hit by a series of unexplained explosions last September, in what Moscow has called an act of "international terrorism".
Denmark, Germany and Sweden have conducted their own investigations into the blasts, but Moscow says it has not been kept informed about the probe.
"We asked the Danish authorities... about a request to work together or to form an international group of experts, specialists," Putin said in an interview with the state Rossiya-1 TV channel.
"The answer, as I said, was vague. Simply put, no answer. They said we had to wait."
Swedish and other European investigators say the attacks were carried out on purpose, but they have not said who they think was responsible. Moscow, without providing evidence, has blamed the explosions on Western sabotage.
Commenting on a report suggesting that a pro-Ukraine had attacked the pipelines, Putin said this was "complete nonsense".
"One should always look for those who are interested. And who is interested? Theoretically, of course, the United States is interested," Putin said.
"An explosion of this kind, of this power, at this depth can only be carried out by specialists, and supported by the full power of the state, which has certain technologies," Putin said.
The United States strongly denies any involvement in the Nord Stream blasts. The White House in February dismissed a blog post by a US investigative journalist alleging Washington was behind explosions as "utterly false and complete fiction."
Separately, Putin said a ship rented by Russian energy company Gazprom had found an antenna-like object about 30 km (19 miles) from the explosion sites.
"Experts believe that this could be an antenna to receive a signal to detonate an explosive device," Putin said.
A letter seen by Reuters on Tuesday showed that Denmark was looking into whether an "object" found close to the only remaining intact Nord Stream gas pipeline under the Baltic Sea posed any safety or environmental risks.
It was not clear what object Denmark was referring to.
"Given the location of the object relevant authorities are currently assessing safety and environmental considerations," the Danish foreign ministry wrote in the letter to the Russian foreign ministry through its embassy in Copenhagen.
On Tuesday, the ministry said in an emailed response to a Reuters query that the assessment of relevant authorities concluded that the object did “not pose any immediate security threat and that there is no immediate threat to marine traffic or people in the area.”