U.S. President Barack Obama says the "ongoing incursion" into Ukraine by Russia will only lead to additional costs for Moscow.
The U.S. leader said Russia is training, arming and funding pro-Russian separatists in Ukraine.
Obama says new images of Russian forces inside Ukraine make that plain to see.
But Obama stopped short of calling Russian involvement in Ukraine an "invasion," and ruled out military action.
Obama said he had spoken with German Chancellor Angela Merkel, and both leaders agreed Russia must face consequences for its actions.
Obama also said Ukraine President Petro Poroshenko will visit the White House next month on September 18.
Obama was speaking to reporters in Washington today shortly after an emergency session at the UN Security Council on the crisis in Ukraine.
Earlier, U.S. Ambassador Samantha Power told an emergency session of the UN Security Council that Russia must "stop lying" about its invasion of eastern Ukraine and its direct military support for pro-Kremlin separatists there.
Power told the UN Security Council on August 28 that Washington will continue working with its partners from the Group of Seven leading industrialized nations "to ratchet up consequences on Russia."
Saying that Russia's "mask is beginning to come off," Power said the Kremlin's "blatant disregard for the United Nations Charter will not be tolerated."
She told the Security Council: "Now, the key question to be answered is not what we can say to Russia to make them hear us, but what we can do."
Urging strong international action against Moscow, Power said that, in the face of the threat now posed by Russia, "the cost of inaction is unacceptable."
The meeting comes after Ukraine's President Petro Poroshenko said on August 28 that Russia has deployed forces in eastern Ukraine.
But Russia's Defense Ministry on August 28 continued to deny that Russian military forces were operating in eastern Ukraine, saying the information "bears no relation to reality."
Ukraine's National Security and Defense Council said Russian tanks and troops seized the border town of Novoazovsk after pummeling government forces with Grad rockets and cross-border artillery fire.
A top NATO official said on August 28 that more than 1,000 Russian soldiers are thought to have invaded eastern Ukraine to help pro-Russian separatists battle government forces, with 20,000 more Russian troops mobilized just across the Russian border.
"They are supporting separatists [and] fighting with them," the officer, who wished to remain anonymous, told reporters in Mons, Belgium.
He said it is conceivable that Moscow could try to create a land bridge linking Russia to the Crimea peninsula, which Russia seized and annexed in March.
WATCH: RFE/RL's Andrei Babitsky shot footage of fierce fighting on the streets of the eastern Ukrainian city of Donetsk on August 28. (WARNING: Graphic content, including dead bodies)
To do so, Russian forces would have to advance further west from Novoazovsk through the town of Mariupol and along the coast of the Sea of Azov through the regions of Zaporizhzhya and Kherson.
Andriy Lysenko, a spokesman for Ukraine's security forces, said on August 28 that government forces had regrouped to defend Mariupol.
He also said Ukrainian troops had clashed with a Russian a military reconnaissance patrol trying to cross into the Kherson region from the Crimean peninsula.
Meanwhile, envoys from all 28 NATO member states have scheduled urgent talks in Brussels on August 29 with Kyiv's NATO ambassador about Russia's military invasion of eastern Ukraine.
The Pentagon says it is deploying Abrams M-1 tanks and armored fighting vehicles to Poland and the Baltic states for a three-month "training exercise."
NATO has already beefed up its aircraft in Poland and the Baltic states as the Ukraine crisis has been unfolding, with Russia taking similar steps with its air forces in the region.
Finland, a member of NATO's Partnership for Peace program, is increasing "reconnaissance flights" after three violations of its airspace by Russian military planes in the past week.
Finland and Sweden, also a partner in the NATO program, say they are signing a pact that allows them to host NATO training exercises and grant greater access to NATO troops on their territory.
In London on August 28, British Prime Minister David Cameron warned of "further consequences" for Moscow unless it halts the advance of Russian tanks into Ukraine.
UN Secretary-General Ban Ki-moon's spokesman, Stephane Dujarric, said that the UN chief is alarmed by reports that the fighting in eastern Ukraine has spread southward.