WASHINGTON (AP) — President Barack Obama suggested Thursday that the U.S. might impose new economic sanctions on Russia, blaming it squarely for the warfare in eastern Ukraine. But he ruled out any military options and proposed no shift in an American-led strategy that has yet to convince Moscow to halt operations against its far weaker neighbor.

Briefing reporters at the White House, Obama said he spoke by telephone with Chancellor Angela Merkel of Germany, Europe’s largest economy and a country that has led diplomatic efforts to end the fighting between Ukraine and Russian-backed rebels. They spoke after two columns of Russian tanks and military vehicles entered the country’s southeast and fired Grad missiles at a border post and 1,000 Russian troops poured into the country, according to NATO and Ukrainian officials.