U.S. officials warned on Sunday that while the diplomatic path to easing tensions between Russia and Ukraine remains open, the window of opportunity for negotiations and dialogue is “shrinking” as Moscow accelerates its military buildup near the Ukrainian border.

Intelligence obtained by the U.S. says that Russia is eyeing Wednesday as the target date for an invasion.

Pentagon press secretary John Kirby said that the “time component” for diplomacy between Russia and Ukraine “seems to be shrinking,” a revelation that he said “gives us all cause for concern.”

The new outlook is materializing as Russia strengthens its military buildup.

National security adviser Jake Sullivan told CBS’s “Face the Nation” that in the previous 10 days or so, the U.S. has observed “a dramatic acceleration in the buildup of Russian forces and the disposition of those forces in such a way that they could launch a military action, essentially at any time.”

During an interview with CNN’s “State of the Union,” Sullivan said Moscow’s movement in recent days has put troops “in a position where they could launch a military action very, very rapidly.”

He emphasized that “a major military action could begin by Russia and Ukraine any day now,” a statement the White House adviser first told reporters on Friday.

And on “Fox News Sunday,” Kirby said “good sources of intelligence” are telling the U.S. that “things are sort of building now to some crescendo opportunity for Mr. Putin.”

Despite the possibility of an invasion becoming more imminent, top U.S. figures are holding out hope for a diplomatic solution.

Sullivan told CNN that the U.S. believes “Russia could choose the diplomatic path,” and Speaker Nancy Pelosi (D-Calif.) told “This Week” on ABC that “let’s hope that diplomacy works.”

The top House Democrat said “an assault on Ukraine is an assault on democracy.”

Kirby, however, said Saturday’s roughly hourlong call between President Biden and Russian President Vladimir Putin, which did not lead to any breakthroughs, did not “give us any cause for optimism.”

“It’s certainly not a sign that things are moving in the right direction. It’s certainly not a sign that Putin has any intention to de-escalate the tensions. And it’s certainly not a sign that he is recommitting himself to a diplomatic path forward,” Kirby added.

The White House said Biden warned Putin of “swift and severe costs” if Russia launches an incursion against Ukraine during a phone call on Saturday, while also noting that the U.S. “remains prepared to engage in diplomacy.”

Biden had a call with Ukrainian President Volodymyr Zelensky Sunday in which he reiterated the U.S. “would respond swiftly and decisively, together with its Allies and partners, to any further Russian aggression against Ukraine,” according to the White House.

Some U.S. lawmakers are also warning about what a Russian invasion could to do Moscow’s relationship with Washington.

Sen. Lindsey Graham (R-S.C.) told ABC that “if Russia invades the Ukraine, you will destroy the U.S.-Russian relationship for decades.”

”And every president in the near term will be put in a box when it comes to dealing with Russia. So I hope Putin understands that,” he added.

The South Carolina Republican also seized the interview as an opportunity to knock the Biden administration for its handling of matters between Russia and Ukraine.

Graham said the U.S. is “talking way too much and we’re doing too little,” while also saying Congress can “do more” amid the escalating situation. He said Congress needs to pass the sanctions bill that has been under negotiations for roughly three weeks.

The coming week will likely be a critical one for the Russia-Ukraine situation in the U.S.