By Arshad Mohammed and David Morgan WASHINGTON, Jan 11 (Reuters) - U.S. Democratic and Republican senators voiced strong concern on Thursday that the Iraq war could spread to neighboring Iran and Syria if the U.S. military were to chase militants across the border. U.S. President George W. Bush, who accuses Iran and Syria of fomenting the violence in Iraq, on Wednesday proposed sending 21,500 more U.S. troops to try to restore security nearly four years after the U.S.-led invasion. Bush sparked worries that the conflict may widen by his comment that "we'll interrupt the flow of support from Iran and Syria. And we will seek out and destroy the networks providing advanced weaponry and training to our enemies in Iraq." U.S. officials said their plan was to disrupt such networks while staying inside Iraq, but their comments did not appear to mollify senior U.S. lawmakers. Senate Foreign Relations Committee Chairman Joseph Biden bluntly told Secretary of State Condoleezza Rice he did not think Bush had the authority to launch attacks to stamp out militant networks in Iran and Syria. "If the president concluded he had to invade Iran ... or Syria in pursuit of these networks, I believe the present authorization granted the president to use force in Iraq does not cover that and he does need congressional authority to do that," said Biden. "I just want to set that marker," added the Delaware Democrat, who later wrote Bush a letter asking for an "authoritative answer" on whether he believed U.S. forces could cross into Iran or Syria without congressional authorization. In a testy hearing about Bush's new plan for Iraq, Rice said she did not want to speculate on the president's constitutional authority for such action. NOTHING RULED OUT "Obviously, the president isn't going to rule anything out to protect our troops, but the plan is to take down these networks in Iraq," she said. Earlier on Thursday, U.S. forces stormed an Iranian government representative's office in the northern Iraqi city of Arbil and arrested five people, including diplomats. "We will continue to work with the Iraqis and use all of our power to limit and counter the activities of Iranian agents who are attacking our people and innocent civilians in Iraq," Rice said in a prepared statement given to the committee. Republican Sen. Chuck Hagel of Nebraska also expressed concern about potential future U.S. action in Iran or Syria."You cannot sit here today -- not because you're dishonest or you don't understand -- but no one in our government can sit here today and tell Americans that we won't engage the Iranians and the Syrians cross-border," said Hagel, a Vietnam veteran and possible 2008 presidential candidate. Several Republican and Democratic senators pressured the Bush administration to talk directly to Iran, but Rice repeated the administration's reluctance to do so unless Iran abandons sensitive atomic work, a step Tehran has so far rejected. Rice suggested Iran might use a dialogue about Iraq to extort U.S. concessions on its nuclear program, a trade-off U.S. officials have rejected. The United States believes the program is aimed at building weapons, while Tehran says it is for generating electricity. The United States has sought to pressure Iran over its nuclear program through a unanimous U.N. Security Council sanctions resolution that passed in December, but the top U.S. intelligence official suggested Iran's economy was resilient. "Record oil revenues and manageable debt suggest that Iran is capable, for now, of weathering shocks to the economy," U.S. intelligence chief John Negroponte said in written testimony submitted to the Senate Select Committee on Intelligence. Separately, key lawmakers warned they would insist that the Bush administration impose sanctions on China if Beijing pursues an agreement to develop Iranian gas fields. The two countries in December announced a preliminary deal, believed worth $16 billion, for China to invest in Iran's north Pars gas field and to construct plants to produce liquefied natural gas. (Additional reporting Sue Pleming)
