WASHINGTON (Reuters) - Secretary of State Hillary Clinton underscored on Thursday the need for international consensus on the next steps on Libya, saying any unilateral US move could have unforeseeable consequences. Clinton also expressed deep doubts about proposals to set up a "no-fly" zone over Libya, saying previous no-fly zones set up over Iraq and Serbia had had little effect. Clinton told the House of Representatives appropriations committee that the Obama administration believed it was imperative that other countries agree on the way forward. "We are working to create an international consensus because we think that is absolutely critical to anything that anybody, especially us, does," Clinton said, saying there was considerable ambivalence over what should be done. Libya is preparing full-scale military action to crush its rebellion and will not surrender even if Western powers intervene in the conflict, Libyan leader Muammar Gaddafi's most prominent son said on Thursday. "It's time for liberation. It's time for action. We are moving now," Saif al-Islam Gaddafi told Reuters in an interview. A reporter from Britain's Guardian newspaper and a Brazilian colleague who went missing in Libya this week were arrested and imprisoned by government forces, Reporters Without Borders said on Thursday. NATO has agreed to continue planning for all military options on Libya, the US defence secretary said on Thursday. South African President Jacob Zuma told Libyan leader Moamer Gaddafi during a recent phone call to end immediately the killing of civilians, foreign minister Maite Nkoane-Mashebane said Thursday. Gaddafi called our president, she told a news conference, though she would not say when the phone call took place. He told Gaddafi how we abhor the heinous violation of human rights against his own people, she said. The top US spy chief said Thursday that better-equipped forces loyal to Libyan leader Muammar Gaddafi were likely to prevail in the long run against rebels fighting to end his 41-year rule. Libya's aerial bombing of civilians and use of heavy weapons on city streets must be investigated as possible crimes against humanity, the top UN human rights official said on Thursday. UN High Commissioner for Human Rights Navi Pillay also said she had received accounts of executions, rapes and disappearances in the north African country. Reports of the "continued aerial bombardment of civilians and the use of military grade weapons and tanks on city streets" were outrageous and "would be investigated as possible crimes against humanity", the former UN war crimes judge said. Libya has descended into civil war with increasing numbers of wounded civilians arriving in hospitals in eastern cities, the International Committee of the Red Cross (ICRC) said. ICRC President Jakob Kellenberger called on Libyan authorities to grant the humanitarian agency access to western areas, including the capital Tripoli, to assess needs. Top Libyan authorities, whom he declined to name, had told him there was no need for outside help in the areas held by Libyan forces, said Kellenberger, a former Swiss diplomat. UN agencies remain shut out from Libya for security reasons, but are increasingly alarmed at sketchy reports of mounting casualties in besieged cities, UN officials said on Wednesday.