"I called their ambassador to the US yesterday and asked for clarification. I am waiting for one," Rice told reporters on her plane prior to making a stopover in Shannon, Ireland, en route to Moscow.
Israel on Tuesday ordered the confiscation of Arab land outside east Jerusalem, officials said, reviving fears that the occupied West Bank could be split in two and challenging peace overtures.
The appropriation orders came as Israelis and Palestinians prepare for a major US-sponsored international peace summit widely expected in Maryland next month, and were immediately criticized by Arab authorities.
Until now, the United States has refused to comment on the Israeli move, with State Department spokesman Sean McCormack saying that he wanted to "understand better the facts on the ground".
Washington's silence contrasted with the reaction of France, Egypt and Jordan, all of which swiftly denounced the land grab.
The situation is likely to complicate matters for Rice when she travels to the Middle East on Sunday following a two-day visit in Russia. The trip had been aimed at laying some of the groundwork for the peace summit.
Her five-day stint in the region is scheduled to take her to Ramallah and Jerusalem, as well as short trips to Amman and Egypt, and is expected to include talks with Israeli Prime Minister Ehud Olmert and Palestinian leader Mahmud Abbas.
'Iran lying about nukes'
Also Thursday, Rice accused Iran of "lying" about the aim of its nuclear program, saying there's no doubt Tehran wants the capability to produce nuclear weapons and has deceived the UN's atomic watchdog about its intentions.
"There is an Iranian history of obfuscation and, indeed, lying to the IAEA," she said, referring to the International Atomic Energy Agency.
"There is a history of Iran not answering important questions about what is going on and there is Iran pursuing nuclear technologies that can lead to nuclear weapons-grade material," Rice told reporters.
US officials have long accused Iran of trying to develop nuclear weapons behind the facade of a civil atomic energy program, charges that Tehran denies. But Rice's strong words, including the blunt reference to Iranian "lying," come at a critical time in dealing with the matter.
The United States is trying to win Russian support for new UN sanctions against Iran but has faced sharp resistance from Moscow, which has nuclear cooperation agreements with Tehran and argues the country should be given more time to come clean on its programs.
Russian President Vladimir Putin said this week there is no proof Tehran is trying to build the bomb. Rice and Defense Secretary Robert Gates are scheduled to see him in Moscow on Friday.
Washington has been pressing for more sanctions since earlier this year.
But last month, the five permanent members of the UN Security Council — Britain, China, France the United States and Russia — and Germany agreed with the support of the European Union to hold off on a new sanctions resolution until November to allow negotiations with Iran to continue.
If no progress is made on two separate tracks — talks with EU foreign policy chief Javier Solana on an offer of assistance in exchange for a suspension in Iran's nuclear program and discussions with IAEA on its past activities — they are to bring the resolution to a vote.
It remains unclear, though, if Russia and China, which also opposes sanctions, will support it.
Even as work on the proposed resolution is to continue at an October 17 meeting of senior diplomats in Europe, Putin said Wednesday that Russia was not convinced Iran is trying to create nuclear weapons.
His comments came after talks with French President Nicolas Sarkozy, whose government is firmly behind the US sanctions drive, and appeared to deal a new blow to efforts to forge a consensus.
"We have no objective data that Iran is seeking to make atomic weapons," Putin said. "Therefore, we proceed from the assumption that Iran has no such plans."
Rice, however, stressed that Russia had signed on to the September 28 agreement to consider new sanctions in November and said she did not "expect that there is any deviation from that course at this point" from the Russian side.
She also noted that Russia had in the past demonstrated its concern about Iran's program by limiting its cooperation to prevent Tehran from acquiring a full nuclear fuel cycle that could be used to produce weapons-grade material.
"That concern was seen very clearly in Russia's offer to Iran to enrich and reprocess in a joint venture and to bring back any spent fuel so that the fuel cycle wouldn't be available to Iran," she said. "I think there is a reason for that and that is suspicion about Iran's intentions."
AP contributed to this report