WASHINGTON (NewsNation Now) —  The United States delivered a written response Wednesday to Russia’s security demands on Ukraine and NATO, Secretary of State Anthony Blinken said.

U.S. Ambassador to Moscow John Sullivan handed over the response to Russia’s Deputy Foreign Minister Alexander Grushko, according to the Associated Press.

The U.S. hasn’t made its response public, but Blinken said the U.S. has made no concessions to the main Russian demands.

Wednesday’s answer to Russia makes very clear that the U.S. is standing by its principles, Blinken said: “There is no change, there will be no change.”

Russia has warned it would quickly take “retaliatory measures” if the U.S. and its allies reject its security demands over NATO and Ukraine.

The Biden administration is set to hold a private, classified briefing Wednesday for U.S. House and Senate leadership aides and staff about the latest intel from the ground in Ukraine, NewsNation has confirmed.

The briefing comes as Russia’s demands have raised pressure on the West amid concerns that Moscow plans to invade its neighbor. Russia has repeatedly denied it has any such designs. But the U.S. and its NATO allies are worried because Russia has massed an estimated 100,000 troops near Ukraine’s border.

On Wednesday, the number two official at the State Department warned an invasion could happen by mid-February.

“We all are aware that the Beijing Olympics are beginning on February 4th, the opening ceremony, and President Putin expects to be there,” U.S. Deputy Secretary of State Wendy Sherman said. “I think that probably President Xi Jinping would not be ecstatic if Putin chose that moment to invade Ukraine. So that may affect his timing and his thinking.”

At the heart of the standoff are questions about Ukraine’s future: Russia has demanded guarantees that NATO will never admit the country and that the alliance will roll back troop deployments in other former Soviet bloc countries. Some of these are nonstarters for NATO, creating a stalemate.

House Speaker Nancy Pelosi and Senate Majority Leader Chick Schumer have requested all-member briefings in each of their chambers. The White House is working to make that happen, as the House and Senate are still on recess.

One of the concerns for lawmakers and staff mostly focuses on the timeline of Russian sanctions. They want to see them implemented now, and some feel delaying sends the wrong signal, not just to Russia but also to China and other foreign leaders.

Their serious concern is that by hesitating, it may not deter Russian President Vladimir Putin from invading if he decides that is what he’s going to do.

“What we don’t want is for further encroachment into Ukraine that would lead us to a situation where suddenly we have Russian troops on the doorstep of Eastern Europe, Poland and other NATO countries,” said Sen. Todd Young, R-Ind.

On Tuesday, President Joe Biden told reporters that Putin “continues to build forces along Ukraine’s border,” and an attack “would be the largest invasion since World War II. It would change the world.” He warned that there would be serious economic consequences for Putin, including personal sanctions, in the event of an invasion.

The Associated Press contributed to this report.