On Thursday, Ukrainian President Petro Poroshenko accused Russia of invading Ukraine as troop movements from across the border were reported for third time this week. His accusations were echoed by both the United States and NATO.

We're keeping track of the developments below.

Update

12:02 p.m.: As with most major events, such as Egypt's coup d'etat or non-coup d'etat in 2013, while the United States is acknowledging the presence of Russian troops in Ukraine, it is refraining from calling it an invasion.

Meanwhile, as 20,000 Russian troops malinger on the other side of the border from Ukraine, Andriy Lysenko, a spokesman for Ukraine's National Security Council, gave some details about the alleged movements of Russian troops earlier today. As the AP reported, military vehicles shot missiles at a border post before crossing into Ukraine and engaging with Ukrainian soldiers:

Lysenko said the missiles from Russia were fired about 11 a.m. and about an hour and a half later, two columns, including tanks and other fighting vehicles began an attack. They entered Ukraine from Veselo-Voznesenka and Maximovo of the Rostov region in Russia. 

9:57 a.m.: Ukrainian calls for the United Nations to address the burgeoning crisis in eastern Ukraine are being heeded. The UN Security Council will reportedly meet later today.

9:42 a.m.: More accusations about Russian military action are coming in over the transom:

Original Post

As Russia reportedly opens a new front in its battle against Ukraine, Ukrainian President Petro Poroshenko became the latest and most notable figure to call Russia's action an invasion of Ukraine. U.S. officials agree, telling CNN that as many 1,000 Russian troops have entered the country and are engaged in the fighting south of Donetsk on Thursday.

This new incursion signals a heightened level of Russian military involvement, and a direct open conflict between the two nations, Moscow continues to deny that they have crossed the border, even though the leader of one of the anti-Kiev rebel groups told Russian television that as many 3,000 to 4,000 Russian soldiers had joined their ranks, claiming they were retired military members or active soldier who chose to join the fight voluntarily while on leave.

Poroshenko's sentiments were echoed by his prime minister:

On Thursday, Poroshenko announced that is canceling a work trip to Turkey in light of the "sharp aggravation." 

From a statement on the presidential website:

I have made a decision to cancel my working visit to the Republic of Turkey due to sharp aggravation of the situation in Donetsk region, particularly in Amvrosiivka and Starobeshevo, as Russian troops were actually brought into Ukraine."

Andrew Kramer and Michael Gordon are reporting on the new offensive by the pro-Russian rebels, anchored a "stealth invasion" by Russian forces:

The latest incursion, which Ukraine’s military said included five armored personnel carriers, was at least the third movement of troops and weapons from Russia across the southeast part of the border this week, further blunting the momentum Ukrainian forces have made in weakening the insurgents in their redoubts of Donetsk and Luhansk farther north. Evidence of a possible turn was seen in the panicky retreat of Ukrainian soldiers on Tuesday from a force they said had come over the Russian border.

Russia continues to deny any military involvement.

As we noted earlier, Poroshenko met with Russian President Vladimir Putin this week, a meeting in which no peace agreement was ironed out between the two leaders. Putin, who also says there is no direct Russian link to the actions of the Russian pro-separatists, put his devil-may-care approach to diplomacy on full display:

 Meanwhile, the death toll is quickly growing in the months' long conflict: