WASHINGTON — U.S. President Barack Obama said on Wednesday Ukraine did not need additional military assistance to help fight insurgents in the eastern part of the country but a Russian invasion would raise "a different set of questions."

Obama's comments came after NATO said on Wednesday Russia had massed some 20,000 troops on Ukraine's border and could use the pretext of a humanitarian mission to invade.

At a news conference at the conclusion of a U.S.-Africa summit in Washington, Obama was asked whether the United States would reconsider sending lethal aid to Ukraine.

"Well keep in mind that the Russian army is a lot bigger than the Ukrainian army. So the issue here is not whether the Ukrainian army has some additional weaponry," Obama said, noting that the separatists had not been able to match the Ukrainian army's strength so far despite causing a lot of violence.

"Now if you start seeing an invasion by Russia, that's obviously a different set of questions. We're not there yet," he said.

The White House said Vice President Joe Biden spoke with Ukrainian President Petro Poroshenko on Wednesday about the situation on the border with Russia.

"The two leaders expressed concern with Russian statements suggesting a role for Russian 'peacekeepers' in Ukraine, with Russia's ongoing military buildup on Ukraine's border, and with Russia's continuing transfer of weapons to Russian proxies in Ukraine," the White House said in a statement.

Obama said the United States would continue to work on a day-by-day, week-by-week basis to determine what Ukraine needed to defend itself against separatists who were backed by Moscow.

Meanwhile, U.S. and European sanctions had succeeded in hurting the Russian economy.

"The economy has ground to a halt," Obama said, noting capital flight from Russia.