UNITED NATIONS — Western diplomats are concerned that an Arab-backed agreement on changing Lebanon’s political system, which was largely seen as a major victory for Syria, Iran, and its Lebanese allies, would end even the façade of an international effort to disarm Hezbollah, allowing the terrorist group to become the de-facto ruler of the country.

Nevertheless, the United Nations Security Council yesterday issued a statement saying it “congratulates the leaders and the people of Lebanon” and “strongly supports” the agreement reached in Doha, Qatar, on Wednesday. The Arab League pact gave Hezbollah enough concessions to end — at least temporarily — its military assault on Beirut and other parts of the country, and to allow the political institutions that the Shiite parties have held hostage for the last 18 months to resume their activity.

America and France, which have backed the pro-Western government of Prime Minister Siniora, showed support for the Doha agreement, although Washington officials expressed regret that it included no reference to disarmament, a key Security Council demand since 2004. Despite that demand, Hezbollah has been able to build a large army that has clashed with Israel and, more recently, attacked Mr. Siniora’s Lebanese allies.

“My concern is that people will confuse Doha, which is a weigh station, with a solution,” an American U.N. ambassador, Alejandro Wolff, said. “Doha is not a solution. Doha is a vehicle by which the Lebanese parties can start building their unity, a functioning government again.” Such unity should then allow implementation of past resolutions, including “ultimately leading to the disarmament of militias,” he said. “One of the core elements of a nation’s sovereignty is a monopoly of the use of force kept in its army, in its military.”

Yesterday’s council statement made a vague reference to “all relevant” past council resolutions, even though America and its allies wanted to add explicit references to the ones that dealt with disarmament, according to diplomats. The French U.N. ambassador, Jean Maurice Ripert, noted that Mr. Siniora and his allies agreed to the Doha agreement. The council could not have moved further than an agreement that is “supported by all the factions” in Lebanon, he said.

As result of the Doha agreement, Lebanon’s parliament is expected to convene on Sunday to elect a new president. The country’s political factions have already agreed to elect the army chief, General Michel Suleiman, who is considered politically independent. His job, however, has required maintenance of close ties with Damascus, which has a lot of influence over Lebanon’s military and intelligence services.

As well-armed Hezbollah operatives ransacked and burned the Beirut offices of Future Television and other assets of Saudi-backed Sunni parties earlier this month, the army declined to intervene and the show of force has deepened the impression that the only viable military force in the country is Hezbollah, according to a diplomat who follows Lebanon closely, and spoke on condition of anonymity.

Shiite politicians allied with Hezbollah, which is backed by Tehran and Damascus, have held up the country’s political process by withdrawing their representatives from Mr. Siniora’s government, and later refusing to convene the parliament. Numerous attempts to convene legislators since the presidential palace was emptied last November had been blocked by the parliament Speaker, Nabih Berri, who leads a Hezbollah-allied Shiite party, Amal.

According to the new agreement, Hezbollah and its allies will control 11 of the government’s 16 seats, giving them enough power to veto all decisions, including those related to cooperation with the international tribunal to try those responsible for Lebanon’s political assassinations. Although Arab leaders are concerned about the growing Iranian influence in the region, they have backed the Doha agreement enthusiastically. “It’s up to the Lebanese,” Qatar’s U.N. ambassador, Nassir Abdulaziz al-Nasser, said yesterday. “They agreed to elect a president and move forward.”