Sarkisian inked the agreement at a meeting of heads of state in the Belarusian capital Minsk.
Last September's Minsk accord involving Russia, pro-Russia rebels and the Ukrainian government included the establishment of a 30km (19-mile) buffer zone between the two sides.
Joe Biden, the US vice-president, "strongly condemned" the violence and warned the "costs to Russia will rise" if it "continues to violate the Minsk agreements, including the most recent agreement signed on 12 February".
Kiev and pro-Russia rebels agreed a peace roadmap on 12 February after marathon negotiations in Minsk involving the leaders of France, Germany, Russia and Ukraine.
When it received a request for extra-judiciary assistance from Minsk in February the public prosecutors in Warsaw promptly supplied details of funds transferred.
When in Minsk, Attenborough asked the actors why they faced such danger.
Attenborough could not wait to return home from Minsk.
Most of Minsk was destroyed in the second world war; it is now an entirely modern city.
The Kremlin condemned the US training mission as a threat to the peace process endorsed by the leaders of France, Germany, Russia and Ukraine in Minsk in February.
"The participation of instructors and specialists from third countries on Ukrainian territory, where the internal conflict remains unregulated, where problems with realising the points of the Minsk agreements persist, definitely doesn't help resolve the conflict," president Vladimir Putin's spokesman, Dmitry Peskov, told journalists after the US paratroopers first arrived in Ukraine on Friday.
"The EU stands ready to take appropriate action in case the fighting and other negative developments in violation of the Minsk agreements continue," she said, making an apparent threat of further economic sanctions on Moscow.
