The US and German leaders say sanctions on Russia must stay until it implements a deal to end fighting in Ukraine. President Barack Obama and Chancellor Angela Merkel held talks as the G7 summit of economic powers began in southern Germany on Monday. Moscow is the target of European Union and US sanctions over its role in support of Ukrainian rebels. Russia has been excluded from what was previously known as the G8, since the annexation of Crimea last year. The West accuses Russia of sending military forces into eastern Ukraine to help the rebels. Moscow denies this, saying any Russian soldiers there are volunteers.