The US state department has issued a security alert warning that Russia is stepping up efforts to launch strikes against Ukraine’s civilian infrastructure and government facilities in the coming days.

The US Embassy in Kyiv urged American citizens still in Ukraine to depart the country immediately.

“If you hear a loud explosion or if sirens are activated, immediately seek cover,” the state department said in its alert.

“If in a home or a building, go to the lowest level of the structure with the fewest exterior walls, windows, and openings; close any doors and sit near an interior wall, away from any windows or openings.”

The state department issued the alert after the US intelligence community on Monday declassified a finding that determined that Russia would increasingly target Ukrainian civilian infrastructure, according to an American official.

The new intelligence comes as Russia’s brutal invasion of Ukraine hits the six-month mark on Wednesday, which also coincides with Ukraine’s independence day from Soviet Union rule.

US President Joe Biden spoke on Sunday with Prime Minister Boris Johnson, French President Emmanuel Macron and German Chancellor Olaf Scholz about concerns about shelling near the Zaporizhzhia nuclear plant in south-eastern Ukraine, and called for the United Nations nuclear watchdog to visit the power plant.

The US official said, however, that the intelligence finding is not specifically tied to concerns about Zaporizhzhia.

The Biden administration is battling to keep western allies – and Washington – focused on maintaining pressure on Russian President Vladimir Putin.

US senate Republican leader Mitch McConnell said on Monday the “single most important thing going on in the world right now is to beat the Russians in Ukraine”.

Speaking at a luncheon event in Scott County, Kentucky, Mr McConnell said the one fear he has is that the US and others will “all kind of lose interest” as the war drags on.

“We need to stick with them,” Mr McConnell said. “It’s important to us, and to the rest of the world, that they succeed.”