Ukraine’s military has briefed key members of parliament that it now believes the situation in eastern Ukraine with Russia may sharply deteriorate in the next two days, according to four sources with knowledge of what was said during the briefing.

Ukrainian President Volodymyr Zelenskyy was present at the Wednesday night briefing, sources said, where the military said they now believe Russia may launch a major attack that would go beyond Eastern Ukraine, targeting at least two major cities. Sources said they told the members of parliament that Kyiv might also be a target -- in line with U.S. officials warning that Russia is preparing a full-scale invasion that will go beyond eastern Ukraine and target Kyiv.

According to two sources, the military believes Russia may target Kharkiv, Ukraine’s second-largest city that is located around 20 miles from the border with Russia in the east, and also Kherson, a city in the south close to Crimea.

In a shift of tone Thursday, Ukraine has been taking new steps to brace for a possible attack, declaring a nationwide state of emergency and calling up 36,000 reservists. But publicly Zelenskyy’s administration has continued to say it is not certain whether Russia will attack. It has said, for now, a full-scale mobilization is not necessary, and it has not declared martial law.

-ABC News' Yulia Drozd and Patrick Reevell

Russia warned Wednesday that the latest sanctions imposed by the United States "will be met with a strong response."

"The round of sanctions announced by the United States Administration [already the 101st in a row] affecting the financial sector with the expansion of the list of persons against whom personal restrictions are imposed is in line with Washington's ongoing attempts to change Russia's course," the Russian Ministry of Foreign Affairs said in a statement. "There should be no doubt that the sanctions will be met with a strong response, not necessarily symmetrical, but well-grounded and sensitive for the American side."

U.S. President Joe Biden announced Tuesday the latest sanctions, which he said would target two Russian banks, Russia's sovereign debt and, starting Wednesday, the Russian elite and their relatives.

-ABC News' Anastasia Bagaeva

Russia marked Defender of the Fatherland Day on Wednesday.

In a video message, Russian President Vladimir Putin congratulated his fellow Russians on the public holiday and noted the importance of ensuring the country's defense capability.

"Dear comrades, today ensuring the defense capability of our country remains the most important state task, and the armed forces serve as a reliable guarantee of national security, the peaceful and calm life of our citizens, and the stable, progressive development of Russia," Putin said.

The Russian leader was seen taking part in a wreath laying ceremony at the Tomb of the Unknown Soldier by the Kremlin wall in Moscow.

-ABC News' Anastasia Bagaeva

Ukraine's military said Wednesday it has begun calling up some reservists in response to an order from Ukrainian President Volodymyr Zelenskyy.

The military general staff said they will be calling up reservists aged 18 to 60 starting Wednesday.

The secretary of Ukraine's National Security and Defense Council, Oleksiy Danilov, said Wednesday that the number of reservists being called up was 36,000, most of whom he said already have combat experience.

On Tuesday, while signing a decree to call up some of Ukraine's military reservists, Zelenskyy emphasized that it was not yet a full mobilization but just the "active reserve," or troops with combat training.

Zelenskyy said the order was necessary because Ukraine's military now needs to be at "heightened readiness" for any changes in the situation on the ground with Russia.

-ABC News' Patrick Reevell