KRUEN, June 7 - Group of Seven (G7) leaders vowed at a summit in the Bavarian Alps on Sunday to keep sanctions against Russia in place until President Vladimir Putin and Moscow-backed separatists fully implement the terms of a peace deal for Ukraine, Reuters reported.

The Ukraine conflict and a long-running debt standoff between Greece and its European partners dominated the first day of the annual meeting hosted by Chancellor Angela Merkel at Schloss Elmau, a luxury Alpine hotel in southern Germany.

Ukraine took center stage on Sunday, with U.S. President Barack Obama calling for "standing up to Russian aggression".

The leaders want Russia and Ukraine to comply with a Feb. 12 ceasefire agreed in the Belarus capital Minsk that largely halted fighting in eastern Ukraine between pro-Russian separatists and Ukrainian government forces.

British Prime Minister David Cameron said Europe had to stay united to keep sanctions on Russia in place despite any pain they caused the 28-nation bloc, urging the Group of Seven industrial nations to send Moscow a tough message on Ukraine.

"We need to make sure Europe remains united," Cameron told reporters at the G7.

"It (sanctions) has an impact on all countries. (But) Britain hasn't let our pre-eminence in financial services get in the way of taking a robust response to Russian-backed aggression and I don't think other countries should either."

EU leaders agreed in March that sanctions imposed over Russia's seizure and annexation of Crimea and detribalization of eastern Ukraine would stay until the Minsk ceasefire was fully applied, effectively extending them to the end of the year, but a formal decision has yet to be taken.

Merkel said any easing of the sanctions depended largely on Russia and its behavior in Ukraine.

European Council President Donald Tusk went further, saying: "If anyone wants to start a discussion about changing the sanctions regime, it could only be about strengthening it."

European monitors have blamed a recent upsurge in violence in eastern Ukraine on the pro-Moscow separatists. Russian President Vladimir Putin was frozen out of what used to be the G8 after the annexation of Crimea in March 2014. (rt/ez)