PARIS - The countries of the European Union have taken their boldest step so far in the increasingly tense standoff with Iran over its nuclear program, agreeing in principle to impose an embargo on Iranian oil, French and European diplomats said on Wednesday. A final decision by the European Union will not come before the end of January and would be implemented in stages to avoid major disruptions in global oil supplies. But the move by some of Iran's most important oil customers appears to underscore the resolve of Western allies to impose toughest round of sanctions on Iran to date, increasing pressure on Tehran to stop enriching uranium and negotiate an end to what Western leaders argue is an accelerating program to build a nuclear bomb. Tehran denies any military intent and refuses to stop enrichment of uranium for what it describes as civilian purposes. But it has responded to the threat of new American and European sanctions by a series of military and diplomatic threats. It has test-fired new missiles, threatened to shut the Strait of Hormuz to shipping, held naval war games, announced the production of its first nuclear-fuel rod and, on Tuesday, warned an American aircraft carrier not to return to the Persian Gulf. It also said that it wanted to reopen talks with the West on the nuclear issue, which was interpreted in Paris as an effort to buy time. The threats from Tehran, aimed both at the West and at Israel, combined with a recent assessment by the International Atomic Energy Agency that Tehran's nuclear program has a military objective, is becoming an important issue in the American presidential campaign. Republican presidential candidates are urging stronger measures against Tehran, including some urging the use of military force, to stop the Islamic government from getting nuclear weaponry and to better protect Israel. Israel itself has warned that time was running out to stop Iran from developing a nuclear weapon, given that Tehran has been moving its enrichment facilities deep into mountains, making it much harder to attack them militarily. Israel has called on Washington and the West to enhance sanctions to a more punishing level in order to get Iran to negotiate seriously and make the development of a nuclear weapon more costly for an economy already suffering from an array of financial and trade sanctions. Last weekend, President Obama signed legislation aimed at sanctioning foreign companies that do business with Iran, which could reduce Iran's ability to sell its oil and other exports. Iran's first vice president, Mohammad-Reza Rahimi, then declared that if sanctions are imposed on Iranian oil exports, Iran would block shipping through the Strait of Hormuz. While American officials discounted the threat as self-defeating for Iran, at the same time they said they had plans to keep the Strait open. Iran has its own domestic politics, with parliamentary elections in March, and the government wants to show that it will protect Iran's interests, both economic and military. The increasingly shrill tone from Tehran seemed a direct response, diplomats suggested, to economic sanctions that are finally biting hard and threaten to damage Iran's ability to export oil. Oil represents some 60 percent of Iran's economy, and oil exports are a vital source of foreign currency. So a European oil embargo would have a limited but significant effect on Iran, which depends heavily on its oil exports for cash to buy needed imports. The United States and France has been pushing hard for an embargo and sanctions targeting Iran's central bank. But some European nations depend heavily on Iranian oil and have been reluctant to cut off their imports during a severe economic slump. At the end of December, lower-ranking diplomats agreed in Brussels to the shape of a European oil embargo on Iran, vowing to meet objections by some states that have significant oil imports from Iran, including Italy, Spain and Greece. James Kanter contributed reporting from Brussels.