NEW YORK — The United Nations Security Council voted unanimously on Wednesday to temporarily rescind part of a 21-year-old arms embargo on Somalia, the East African country once considered synonymous with warlordism, piracy and anarchy, portraying the easing as part of an effort to strengthen the fragile Somalian government’s authority by permitting it to purchase light weapons. A resolution adopted by the council also reauthorised, for one year, the African Union Mission to Somalia, a force of nearly 18,000 peacekeepers deployed in the country. Those troops are considered largely responsible for helping bring a measure of calm and stability to Somalia over the past 18 months by retaking territory formerly held by the Shabab, a fearsome Islamist militant group affiliated with Al Qaeda. President Hassan Sheikh Mohamed, who took office six months ago and has been strongly supported by the United States and Britain, had been pressing for an end to the United Nations arms embargo, arguing that it was outdated and impeded his ability to further empower Somali forces by ensuring that Shabab militants and their affiliates remain in retreat. But a number of Security Council members worried that terminating the embargo would once again allow all manner of weapons to flow into the country, and not necessarily into the government’s hands. There was also concern among some diplomats that the Somali military remains little more than an amalgam of former militia members who are not so disciplined as to warrant unfettered access to weapons. The resulting language in the Security Council resolution, which was drafted by Britain, reflected a compromise.