In early 1854, the Russians again advanced by crossing the River Danube into the Turkish province of Dobruja. By April 1854, the Russians had reached the lines of Trajan's Wall, where they were finally halted. In the centre, the Russian forces crossed the Danube and laid siege to Silistra from 14 April with 60,000 troops. Sustained Ottoman resistance had allowed French and British troops to build up a significant army in nearby Varna. Under additional pressure from Austria, the Russian command, which was about to launch a final assault on the fortress town, was ordered to lift the siege and retreat from the area, thus ending the Danubian phase of the Crimean War.