WASHINGTON (AP) — The U.S. worked Sunday to ramp up diplomatic and financial pressure on Russia over Ukraine, promising to put Moscow on the defensive at the U.N. Security Council as lawmakers on Capitol Hill said they were nearing agreement on "the mother of all sanctions."

The American 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 fears it is planning an invasion.

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

Ukraine's ambassador to the U.S., Oksana Markarova, warned that Russian President Vladimir Putin is bent on waging an "attack on democracy," not just on a single country. It's a case that some senior foreign policy figures have urged President Joe Biden to make, including at the Security Council.

"If Ukraine will be further attacked by Russia, of course they will not stop in Ukraine," Markarova said on CBS's "Face the Nation."

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

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. NATO and the U.S. call those demands impossible.

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."

Ukraine's foreign minister, Dmytro Kuleba, countered that on Twitter, saying: "If Russian officials are serious when they say they don't want a new war, Russia must continue diplomatic engagement and pull back military forces."

The United States and European countries say a Russian invasion would trigger heavy sanctions.

On Sunday, the chairman of the Senate Foreign Relations Committee, Sen. Bob Menendez, said that in the event of an attack, lawmakers want Russia to face "the mother of all sanctions." That includes actions against Russian banks that could severely undermine the Russian economy and increased lethal aid to Ukraine's military.

The sanctions under consideration would apparently be significantly stronger than those imposed after Russia annexed Crimea from Ukraine in 2014. Those penalties have been seen as ineffective.

Menendez also raised the prospect of imposing some punishments preemptively, before any invasion.

"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," the New Jersey Democrat said on CNN.

The desire to hit Russia harder financially over its moves on Ukraine has been a rare area of bipartisan agreement in Congress. But Republicans and Democrats have been divided over the timing of any new sanctions package.

Many GOP members are pushing for the U.S. to impose tough penalties immediately instead of waiting for Russia to send new troops into Ukraine. The Biden administration and many Democratic lawmakers argue that imposing sanctions now against Putin would remove any deterrent to invasion.

Sen. James Risch of Idaho, the ranking Republican on the Foreign Relations Committee, told CNN he is "more than cautiously optimistic" that Republicans and Democrats will be able to resolve their differences over the timing of sanctions.

Russia has long resented NATO's granting of membership to countries that were once part of the Soviet Union or were in its sphere of influence as members of the Warsaw Pact.

NATO "has already come close to Ukraine. They also want to drag this country there," Russian Foreign Minister Sergey Lavrov said on Sunday, "although everyone understands that Ukraine is not ready and could make no contribution to strengthening NATO security."

Ukraine has sought NATO membership for years, but any prospects of joining appear far off as the country struggles to find political stability and attack corruption.

Sen. Dick Durbin, an Illinois Democrat and member of the Senate's Ukraine Caucus, suggested that Ukraine's backing off its NATO aspirations could expedite a diplomatic solution to the current crisis.

If Ukrainian President Volodymyr Zelenskyy "decides that the future membership, if there's to be one in NATO for Ukraine, and the question of the Russian occupation of Ukraine are two things to put on the table, I think we may move toward a solution to this," Durbin said on NBC.

Ukraine has not shown signs of willingness to make concessions on potential alliance membership. It is not clear whether Durbin's suggestion has broader backing.

Lavrov also underlined Russia's contention that NATO expansion is a threat, saying the alliance has engaged in offensive actions outside its member countries.

"It is difficult to call it defensive. Do not forget that they bombed Yugoslavia for almost three months, invaded Libya, violating the U.N. Security Council resolution, and how they behaved in Afghanistan," he said.

The U.S. and NATO have formally rejected Russia's demands about halting NATO expansion, though Washington outlined areas where discussions are possible, offering hope there could be a way to avoid war.

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Heintz reported from Moscow.