Can the UN do more than just talk about Russia, Ukraine crisis?

US Ambassador to the United Nations, Linda Thomas-Greenfield, canceled a planned visit to Liberia this week to focus instead on UN diplomacy on Russia's troop build-up near Ukraine as Washington warns Moscow could invade at any time.

Tensions over the Ukraine crisis spilled into the UN Security Council two weeks ago at a public meeting requested by Washington. Russia, which denies planning an invasion, made a failed bid to stop the council discussion.

While all members were able to air their views openly, there was no action by the 15-member council and will not be - even if Russia invades Ukraine. A simple statement needs consensus support and Russia could veto any bid for a resolution.

Russia is one of five permanent, veto-wielding powers on the council along with the United States, France, Britain and China. The Security Council is charged with maintaining international peace and security.

Russia also currently holds the council's rotating presidency for February.

What happened with Crimea?

If Russia's military escalates the crisis, diplomats and foreign policy analysts say diplomacy and action at the United Nations is likely to mirror what happened in 2014 after Russia annexed Ukraine's Crimea region.

The Security Council has met dozens of times since then to discuss the Ukraine crisis. In March 2014, it voted on a US-drafted resolution opposing a referendum on the status of Crimea and urging countries not to recognize it. It received 13 votes in favor, China abstained and Russia cast a veto.

Seeking to demonstrate Russia's international isolation, Western countries then took a similar text to the 193-member General Assembly, which adopted a resolution declaring the referendum invalid. It received 100 votes in favor, 11 against and 58 abstentions, while two dozen countries didn't vote.

General Assembly resolutions carry political weight but are not legally binding. Unlike the Security Council, no country has veto power in the General Assembly.

What are western diplomats doing? 

So far, Western diplomacy at the United Nations during the latest military build-up has largely focused on trying to rally support - should they need it - among UN members by accusing Russia of undermining the UN Charter.

The Charter is the founding document of the United Nations, outlining its purposes and principles agreed in 1945.

"Russia's actions toward Ukraine are not only a regional issue," Thomas-Greenfield said last month.

"They impact every UN member state, and we must be prepared to stand together in unity and solidarity should Russia defy the shared values and principles that undergird our international system," she said.

What can Russia do as security council president? 

Russia holds the council's rotating presidency for February. This is largely an administrative role, but does involve scheduling meetings, so some diplomats warn Russia could delay any attempts by council members to request another discussion on actions by Russia.

As things stand, the council is already due to discuss Ukraine on 17 February, diplomats said. It is a regularly scheduled meeting on the Minsk agreements, which were endorsed by the council in 2015 and designed to end a separatist war by Russian-speakers in eastern Ukraine.

The General Assembly is also set to hold an annual discussion on "the situation in the temporarily occupied territories of Ukraine" on 23 February.