US State Department Spokesperson Ned Price on Wednesday said the US had communicated its position regarding Russia's "renewed invasion of Ukraine" to Pakistan, adding that it was the responsibility of every country to "voice objection" to Russian President Vladimir Putin's plans.

He was responding to a question during a briefing about how the US saw Prime Minister Imran Khan's visit to Russia amid tensions between Moscow and Ukraine, which has since escalated to an armed conflict.

"We've communicated to Pakistan our position regarding Russia's further renewed invasion of Ukraine, and we have briefed them on our efforts to pursue diplomacy over war," Price said, according to a transcript available on the State Department website.

Prime Minister Imran arrived in Russia late on Wednesday on a two-day visit to push for the construction of a long-delayed, multi-billion-dollar gas pipeline to be built in collaboration with Russian companies.

The prime minister's trip to meet President Vladimir Putin and discuss issues including economic cooperation came hours after a number of Western nations hit Russia with new sanctions for its military deployment into parts of eastern Ukraine.

During a briefing on Wednesday, Price was asked by a reporter what the State Department's assessment was of the timing of PM Imran's visit.

Saying the US was "certainly aware" of the trip, the spokesperson said, "We believe it's the responsibility of every responsible country around the world to voice concern, to voice objection to what Putin appears to have in mind for Ukraine."

Price said the US viewed its longstanding partnership and cooperation with Pakistan as critical to America's interests.

"We certainly hope, when it comes to those shared interests — the aversion of a costly conflict, the aversion of a destabilising conflict, that every country around the world would make that point clearly in unambiguous language in their engagements with the Russian Federation," he added.

When asked whether the US believed PM Imran's visit was an "indirect endorsement" of Putin, Price said the reporter would have to ask the Pakistani government what its intent was.

"I'm just not in a position to offer an assessment on the timing of foreign counterparts' travel to another country," he said.

Read: 'Tread with caution' — Analysts comment on PM Imran's Russia trip amid Ukraine tensions

In an interview ahead of his trip, PM Imran had expressed concern about the situation in Ukraine and the possibility of new sanctions and their effect on Islamabad's budding cooperation with Moscow.

On Thursday, Russian President Putin announced a military operation in Ukraine with explosions heard soon after in the capital and other parts of the country, prompting outrage from US President Joe Biden who warned of a "catastrophic loss of life".

"I have made the decision of a military operation," he said in a surprise statement on television shortly before 6am (0300 GMT).

Putin warned other countries that any attempt to interfere with the Russian action would lead to consequences they have never seen.