As of 22 September 1999, Deputy Interior Minister Igor Zubov said that Russian troops had surrounded Chechnya and were prepared to retake the region, but the military planners were advising against a ground invasion because of the likelihood of heavy Russian casualties. The Chechen conflict entered a new phase on 1 October 1999, when Russia's new Prime Minister Vladimir Putin declared the authority of Chechen President Aslan Maskhadov and his parliament illegitimate. At this time, Putin announced that Russian troops would initiate a land invasion but progress only as far as the Terek River, which cuts the northern third of Chechnya off from the rest of the republic. Putin's stated intention was to take control of Chechnya's northern plain and establish a "cordon sanitaire" against further Chechen aggression; he later recalled that the cordon alone was "pointless and technically impossible," apparently because of Chechnya's rugged terrain. According to Russian accounts, Putin accelerated a plan for a major crackdown against Chechnya that had been drawn up months earlier. The Russian army moved with ease in the wide open spaces of northern Chechnya and reached the Terek River on 5 October 1999. On this day, a bus filled with refugees was reportedly hit by a Russian tank shell, killing at least 11 civilians; two days later, Russian Su-24 fighter bombers dropped cluster bombs on the village of Elistanzhi, killing some 35 people. On 10 October 1999, Maskhadov outlined a peace plan offering a crackdown on renegade warlords; the offer was rejected by the Russian side. He also appealed to NATO to help end fighting between his forces and Russian troops, without effect. On 12 October 1999, the Russian forces crossed the Terek and began a two-pronged advance on the capital Grozny to the south. Hoping to avoid the significant casualties that plagued the first Chechen War, the Russians advanced slowly and in force, making extensive use of artillery and air power in an attempt to soften Chechen defences. Many thousands of civilians fled the Russian advance, leaving Chechnya for neighbouring Russian republics. Their numbers were later estimated to reach 200,000 to 350,000, out of the approximately 800,000 residents of the Chechen Republic. The Russians appeared to be taking no chances with the Chechen population in its rear areas, setting up "filtration camps" in October in northern Chechnya for detaining suspected members of "bandformirovaniya" militant formations (literally: "bandit formations"). On 15 October 1999, Russian forces took control of a strategic ridge within artillery range of the Chechen capital Grozny after mounting an intense tank and artillery barrage against Chechen fighters.