This post updated at 10:15 a.m. ET.
A Ukrainian government spokesman says one of its warplanes was shot down in the country's east by a Russian air force jet, as the U.S. and Europe stepped up sanctions on Moscow over its support of separatist rebels.
Ukraine says a ground-attack Su-25 was downed by an air-to-air missile Wednesday evening over the eastern region of Luhansk. The pilot reportedly safely ejected before the plane crashed. Vitaly Churkin, Russia's ambassador to the United Nations, denied that his country was responsible for the downed plane.
Reuters notes that Kiev's comment on the incident is "the strongest Ukrainian allegation to date of direct Russian military involvement in the conflict."
Separately, another Ukrainian Su-25 was hit by a rebel missile, but suffered only slight damage and landed safely, according to Ukrainian officials.
In another incident last week, Ukraine said one of its AN-26 transports was shot down by a ground or air-launched missile fired from Russia. Two of the eight crew members were killed.
According to the BBC:
"[Pro-Kiev] activists have pointed to videos which appear to show Grad multiple rocket launchers being fired from Russian soil in the direction of Ukraine.
"Nato says that Russian troop numbers on the border have increased again to about 12,000."
Meanwhile, the White House announced Wednesday that it was stepping up sanctions against Moscow in hopes of pressuring President Vladimir Putin to end his support of pro-Russian separatists.
In Mosow, NPR's Corey Flintoff says the sanctions are already having an effect, with share prices opening sharply lower on Thursday in Russia's major stock exchange. European leaders agreed Wednesday to impose similar sanctions. The Moscow Times says that decision "is a significant ratcheting-up of European pressure on Russia although it falls short of the hard-hitting economic measures against Russia for which the U.S. and hawks in the EU were pushing."
"The U.S. moves to impose restrictions on the Russian state-controlled oil giant OAO Rosneft and other top firms are aimed at squeezing Russia's already struggling economy and financial system. They followed weeks of U.S. threats that Russia would face repercussions unless it helped defuse the crisis in eastern Ukraine, where pro-Russia separatists have been fighting the Ukrainian government for months.
"Mr. Putin bristled at the new sanctions. 'They tend to have a boomerang effect, and without a doubt, in this case they have driven Russian-American relations to a dead end, causing very serious damage,' he said. 'I am convinced that is to the detriment of the long-term national interests of the American government and its people.' "