One of the US State Department's top officials said Wednesday that Iran had made only "modest" progress on its nuclear program because of hindrance by United Nations-imposed sanctions. "While Iran seeks to create the perception of advancement in its nuclear program, real progress has been more modest," Undersecretary of State for Political Affairs William Burns, who holds responsibility for the Iran portfolio, told the US House Foreign Affairs Committee. "It's apparent that Iran has not yet perfected [uranium] enrichment and, as a direct result of UN sanctions, Iran's ability to procure technology or items of significance to its missile programs, even dual-use items, is being impaired." He also said "it's hard not to conclude that the Iranians are pursuing a nuclear weapons capability," which he based in part on the Islamic Republic's pursuit of fissile material and the progress it had been making on missile systems, such as those tested earlier Wednesday. To deal with this threat, he said, "we view force as an option that is on the table, but a last resort." He also stressed that "we do not believe we have exhausted all the diplomatic possibilities." To that end, he was pressed by members of Congress to impose sanctions against foreign companies investing large amounts in Iran, in accord with legislation passed by Congress several years ago. Rep. Brad Sherman (D-California) pointed out that in the law's seven-and-a-half years of existence, some 50 examples of companies making such investments had been revealed in the media and at committee hearings. While Burns said that no sanctions against foreign companies had been imposed to date, he indicated that the United States was reviewing a recent Iranian deal with Norway's Statoilhydro to see if it merited becoming the first company to face these sanctions. In the past, the Bush administration has generally used the law's waiver rather than impose penalties on foreign companies. Burns added that the threat of sanctions had contributed to other companies pulling back from deals they had been contemplating with Iran. The committee members also questioned why American exports to Iran - most of which are banned - have been increasing at the same time that the US has been trying to squeeze Iran financially. The Associated Press reported Tuesday that the value of US exports to Iran had grown significantly during US President George W. Bush's years in office - from about $8 million in 2001 to nearly $150 million last year. The exports, made under agricultural, medical and humanitarian exemptions to US trade sanctions, included cigarettes, bull semen, corn, soybeans and medicine, among other goods. Rep. David Scott (D-Georgia) told Burns this was a "sort of schizophrenic approach" to US trade with Iran. Burns replied that the US government's quarrel is with the government of Iran, not with its people, and that American shipments account for a tiny fraction of Iran's total imports. In his testimony, Burns described the behavior of the Iranian regime as posing "as serious a set of challenges to the international community as any problem we face today." HFAC chairman Howard Berman (D-California) went even further, saying in his opening statement that "stopping Iran's nuclear quest is our most urgent strategic challenge." In light of that threat, HFAC Middle East subcommittee chairman Gary Ackerman (D-New York) and ranking member Mike Pence (R-Indiana) have sponsored a non-binding resolution calling on the Bush administration to lead an international effort prohibiting the export to Iran of all refined petroleum products as well as the international movement of all Iranian officials not involved in negotiating the suspension of Iran's nuclear program, among other actions. Some members of Congress, like Barbara Lee (D-California), who challenged the resolution at the hearing, and groups including Americans for Peace Now have opposed the measure out of a concern that it would be seen as giving approval for the administration to take bellicose measures like blocking the Strait of Hormuz in order to prevent Iran from receiving petroleum products. But more than half of House members have co-sponsored the measure according to Ackerman, who defended it as in no way intended to be read as a declaration of war or calling for a naval blockade. "The whole idea that the resolution calls for a blockade can only be sustained by a determined refusal to read the resolution or accept the plain meaning of the words within it," he said. "Sanctions measures are an attempt to avoid war, not to start it." The Associated Press contributed to this report.