The Ukrainian military began training Monday with a group of approximately 300 U.S. Army paratroopers who traveled to Ukraine to work with the country’s national guardsmen. Ukraine President Petro Poroshenko greeted the troops at a military base in the western city of Lviv.

Poroshenko said Ukraine must revamp its military training given the nation’s tense international dealings, the Associated Press reported. Ukrainian government forces have battled pro-Russia separatist rebels purportedly backed by the Kremlin since last April. More than 6,000 people have died as a result of the conflict in eastern Ukraine.

Paratroopers from the U.S. Army’s 173rd Airborne Brigade arrived in Ukraine last week and will reportedly train 900 members of Ukraine’s national guard over the next six months as part of Operation Fearless Guardian. Ukraine reconstituted its national guard last year to consist of former members of local militias, as well as volunteers. The U.S. said it would not supply weapons or lethal training to the guardsmen.

“We will be conducting classes on war-fighting functions, as well as training to sustain and increase the professionalism and proficiency of military staffs,” U.S. Army Maj. Jose Mendez, operations officer of the 173rd brigade, said in a statement.

Russian leaders have expressed concern regarding the U.S. training exercises, which they said could upset a delicate ceasefire that has existed between Ukraine’s military and the pro-Russian separatists since February. “The participation of instructors or specialists from third countries on Ukrainian territory, where the domestic Ukrainian conflict is unresolved … could destabilize the situation,” Russian government spokesman Dmitry Peskov said, Reuters reported.

Western nations and the NATO military alliance have repeatedly accused Russia of providing weapons and direct support to rebels in eastern Ukraine. The Kremlin has denied direct involvement in the conflict.