The Prime Minister was updated on the latest situation in the Baltic Sea in talks with her Danish counterpart Mette Frederiksen in Downing Street on Saturday.
The blasts last week occurred as Russian president Vladimir Putin was preparing to announce the annexation of four Ukrainian provinces – a move widely denounced in the West as a violation of international law and a serious escalation of the conflict.
There are deep suspicions in Western capitals that Moscow carried out the attacks on the pipelines, which can carry gas to Germany, as a way of intensifying the pressure over energy supplies.
However, in a speech on Friday, the Russian leader claimed “Anglo Saxons” were responsible for the “terror attacks”, which he described as an attempt to “destroy the European energy infrastructure”.
The US State Department has dismissed such claims as “preposterous” and “Russian disinformation”.
Following Ms Truss’s meeting with Ms Frederiksen, a No.10 spokesman said: “The leaders stressed the need to stay united in the face of Russia’s despicable action in Ukraine.
“They agreed the incidents were clearly an act of sabotage. The Prime Minister offered the UK’s support for the ongoing investigation.”
Speaking outside No.10, Ms Frederiksen said: “One of the reasons why I’m here today is because of the situation in the Baltic Sea, with Nord Stream 1 and 2.
“I was able to give some details about what has happened in Denmark, or just outside Denmark.
“Of course, it has been very important for me to underline that the Danish authorities have said that this is not an accident.
“This is sabotage and it is critical infrastructure.”
Neither leader sought to attribute responsibility for the incident.
The Danish and Swedish governments last week reported a series of four explosions in the two pipelines, which resulted in a large leakage of methane gas into the Baltic.