In 1914, British forces from India occupied Iranian territory east of the Shatt al-Arab waterway, to guard oil concessions in Iran and in 1915 advanced up the Tigris river to Ctesiphon near Baghdad, before being defeated by the Ottoman army and forced to retreat to Kut. During the Siege of Kut the Ottomans defeated three relief attempts and refused an offer of £2,000,000 to ransom the garrison, which surrendered at the end of April. Hopes of a Russian relief force from the Caspian Sea through Kermanshah and Khanaqin (Khanikin) to Baghdad failed to materialise. From May–November 1916, the British consolidated a hold on Ottoman territory to the west of Iran around Basra and the confluence of the Euphrates and Tigris at the head of the Persian Gulf. In 1917 the British campaign in Mesopotamia continued, with advances to Baghdad and towards the oilfields of Mosul as the campaign in the Levant led to the occupation of Palestine. A Russian invasion of 1915 from Caucasus established bases at Resht, Kazvin and Teheran and led to inconclusive operations between the Russians and Ottomans further west, closer to the Iranian–Ottoman border.