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."