Russian President Vladimir Putin attends a meeting. File photo

The US sought to step up pressure on Russia over Ukraine on Sunday, promising to put Moscow on the defensive at the UN Security Council as lawmakers on Capitol Hill said they were nearing agreement on "the mother of all sanctions.”

The US ambassador to the United Nations said the Security Council will press Russia hard in a Monday session to discuss its massing of troops near Ukraine and rising fears it is planning an invasion.

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Any formal action by the council is extremely unlikely given Russia's veto power and its ties with others on the council, including China. But the US referral of Russia's troop buildup to the United Nations' top body gives both sides a big stage in their battle for global opinion.

US Ambassador to the United Nations, Linda Thomas-Greenfield speaks to reporters. File/AP

"Our voices are unified in calling for the Russians to explain themselves,” Ambassador Linda Thomas-Greenfield said of the US and the other council members on ABC's "This Week” on Sunday. ”We're going into the room prepared to listen to them, but we're not going to be distracted by their propaganda."

Russian President Vladimir Putin has made no public remarks about the Western response. Russian Foreign Minister Sergey Lavrov has said the West's position leaves little chance for reaching agreement, though he also said Russia doesn't want war.

Russia's massing of an estimated 100,000 troops near the border with Ukraine has brought increasingly strong warnings from the West that Moscow intends to invade. Russia is demanding that NATO promise never to allow Ukraine to join the alliance, and to stop the deployment of NATO weapons near Russian borders and roll back its forces from Eastern Europe.

Russian Foreign Minister Sergey Lavrov speaks during an event. File photo

The head of Russia's Security Council, Nikolai Patrushev, on Sunday rejected Western warnings about an invasion.

"At this time, they're saying that Russia threatens Ukraine — that's completely ridiculous,” he was quoted as saying by state news agency Tass. "We don't want war and we don't need it at all."

The United States and European Union countries say a Russian invasion would trigger heavy sanctions. On Sunday, the chairman of the Senate Foreign Relations Committee, Sen. Bob Menendez, raised the prospect of imposing some punishments preemptively.

Chairman Bob Menendez speaks during a hearing of the Senate Foreign Relations in Washington. File/AP

Congressional Republicans and Democrats have been divided over the timing of possible sanctions, with many GOP members pushing for the US to impose tough penalties immediately instead of waiting for Russia to send new troops into Ukraine.

"There are some sanctions that really could take place up front, because of what Russia's already done — cyberattacks on Ukraine, false-flag operations, the efforts to undermine the Ukrainian government internally,” Menendez said on CNN.