US Ambassador Zalmay Khalilzad exchanged sharp remarks with the Russian ambassador on Sunday, accusing Moscow of resisting attempts to make peace with Georgia after days of fighting have left hundreds of civilians dead. Khalilzad pointedly asked Russian Ambassador Vitaly Churkin in the UN Security Council session whether Russia's aim was to "change the leadership in Georgia" - a charge Churkin did not directly address but seemed to deny. "We believe the situation has gotten a lot worse in the last 24 hours," Khalilzad told reporters after the public debate during the council session had ended. "There is a danger here that Russia is over-reaching" by seeking regime change in Georgia. "Regime change is purely an American invention," Churkin replied, speaking to reporters. "He [Khalilzad] raised the issue and I think I responded quite adequately to it." The Security Council met Sunday for the fourth time in as many days trying to resolve a conflict that began when US-allied Georgia tried to control South Ossetia, then retreated in the face of Russia's tanks and aircraft. Russia is "ready to put an end to the war," said Russian Ambassador Vitaly Churkin, who also accused the UN secretary-general's office of taking Georgia's side in the dispute. However, Russia also has deployed a naval squadron off the coast of Abkhazia, another separatist region in Georgia, and its aircraft bombed the outskirts of Tbilisi, the Georgian capital. Georgia's Foreign Ministry said its soldiers were observing a cease-fire on orders of the president and declared the move in a note handed over to Russia's envoy to Tbilisi. "They're ready for immediate talks with the Russian Federation," confirmed UN Undersecretary-General for Political Affairs B. Lynne Pascoe during a briefing to the Security Council. He said Georgia's "humanitarian corridor" for civilians, refugees and troops would help facilitate the negotiations. Responding to Churkin's remarks, British Deputy Ambassador Karen Pierce defended UN Secretary-General Ban Ki-moon's office as impartial. "An attack on the Secretariat is an attack on the institution," she said. "It does it no credit." She questioned why Russia was not willing to agree to an immediate cease-fire, and called on Russia to use its influence in Abkhazia to control forces seeking to widen the conflict. US Ambassador Zalmay Khalilzad also accused Russia of resisting attempts to make peace. Many of the council members side with Georgia, which is not a council member. Georgia's ambassador, who was present Sunday, can only join the council's open meetings, not its private talks, and then only by invitation. But Russia is one of five nations with veto power on the 15-nation council. The others are Britain, China, France and the US. "The time has come for us all to show our responsibility and to end a deteriorating process," said French Deputy Ambassador Jean-Pierre Lacroix, whose nation holds the European Union presidency for the rest of this year. Earlier Sunday, Jim Jeffrey, US President George W. Bush's deputy national security adviser, said the US made it clear that "if the disproportionate and dangerous escalation on the Russian side continues, that this will have a significant long-term impact on US-Russian relations." Bush on Saturday called for an end to the Russian bombings and an immediate halt to the fighting, accusing Russia of using the issue of South Ossetia to bomb other regions in Georgia. Also Sunday, Pope Benedict XVI urged an immediate end to hostilities in South Ossetia and the start of negotiations between Russia and Georgia over the contested province. Following a weekly Sunday prayer, Benedict expressed "profound anguish" over the fighting in Georgia and its breakaway province, deploring that the conflict "has already caused a great number of innocent victims and forced a great number of civilians to leave their homes." He made an appeal for peace in the name of the common Christian roots shared by the Roman Catholic Church and Orthodox Christians. A great majority of Georgians and Russians are Orthodox. Calling for "an immediate end to military actions," the pope urged all sides to "refrain, also in the name of the common Christian heritage, from further confrontations and violent retaliations that could degenerate into a wider conflict." The pope urged the international community and influential countries to "make every effort to support and promote initiatives aimed at reaching a peaceful and lasting solution, in favor of an open and respectful coexistence." He said Roman Catholics were joining Orthodox Christians in praying for such an outcome. Benedict spoke to a crowd of 9,000 gathered in the main square in front of the twin-towered cathedral of Bressanone, a town in the Italian Alps where the pontiff is spending a two-week vacation.