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US flies reconnaissance aircraft over eastern Ukraine

(CNN) The US Air Force flew a reconnaissance mission this week over eastern Ukraine to gather intelligence about the situation on the ground -- two days after Russia announced it would withdraw 10,000 troops from its border with that nation.

Monday's flight was the first time an E-8C Joint Surveillance Target Attack Radar System (JSTARS) aircraft has flown over the region, according to Lt. Cmdr. Russ Wolfkiel, a spokesman for US European Command.

While the Pentagon has not revealed exactly what intelligence the aircraft was gathering, Wolfkiel told CNN that the US "routinely operates aircraft in the Black Sea region in support of various US and coalition intelligence objectives."

The aircraft flew with the permission of the Ukrainian government, and while the exact route has not been revealed, Wolfkiel said it stayed out of the Donbas region where pro-Russian separatist forces demanding independence from Ukraine operate.

Still, the aircraft sensors can collect intelligence about military movements on the ground, potentially on both sides of the border, without entering disputed areas or Russian airspace.

An on-board antenna has a 120-degree field of view that can cover nearly 20,000 square miles to monitor ground movements, according to an Air Force fact sheet, and can also detect aircraft.

The developments come as tensions continue to rise given Russia's military buildup on its border with Ukraine, which has led the US to urge de-escalation and warn of economic sanctions should Russia invade its neighboring country. US intelligence findings have estimated that Russia could begin a military offensive in Ukraine "as soon as early 2022" as it amasses up to 175,000 troops along the border. The Russian Ministry of Defense claimed in a statement Saturday that 10,000 troops had return to their permanent bases after completing training near the Ukrainian border.

CNN reported Tuesday that Defense Secretary Lloyd Austin ordered the aircraft carrier Harry S. Truman and its escort ships to stay in the Mediterranean area rather than sail to the Middle East as part of an effort to reassure European allies of the US military's commitment to defense of the region.

The move will keep US warships in the region during upcoming US and Russian talks as well as expected Russian talks with NATO next month.

President Joe Biden on Monday signed into law the annual military funding bill that includes $300 million in military aid to the Ukrainian Security Assistance Initiative, which provides training, equipment and support to Ukraine's efforts to secure its borders.

Biden also plans to hold a call with his Russian counterpart Vladimir Putin on Thursday afternoon "to discuss a range of topics, including upcoming diplomatic engagements with Russia," National Security Council spokesperson Emily Horne told CNN on Wednesday.

CNN's Veronica Stracqualursi contributed to this report.