Insurgents control most of Somalia after a year of intense fighting
Eritrea has labelled UN sanctions imposed on Wednesday as shameful, and denied allegations that it arms Islamist militants in Somalia.
Eritrea's UK ambassador Tesfamichael Gerahtu told the BBC that the sanctions were illegal and would only worsen the problems in the Horn of Africa.
The Security Council imposed an arms embargo, travel bans and asset freezes on top Eritrean officials.
Somalia's beleaguered UN-backed government welcomed the sanctions.
Islamist insurgents have asserted control over most of the country, leaving the government with authority in only small parts of the capital, Mogadishu.
But Mr Tesfamichael said the accusations made by the country's critics were inconsistent.
"Originally it was said we had soldiers and then later came military support and now all of a sudden after certain discussions and opposition they started to talk about political, military and logistical support," he told the BBC's World Today programme.
"Now we are 100% sure that we have never, never, never supplied military equipment or otherwise to the extremists in Somalia."
Eritrea's neighbouring countries and regional blocs including the African Union had been lobbying for sanctions for most of the year.
The resolution demands that the country stops "arming, training and equipping armed groups and their members, including al-Shabab, that aim to destabilise the region".
As a result of the Security Council vote, Eritrea becomes the first new country to be subjected to UN sanctions since they were imposed on Iran in 2006.
The US said it had sought talks with Eritrea for months, but the country had failed to act on its promises.
The UN has frequently expressed concern about the flow of arms in to Somalia, where hard-line Islamists of al-Shabab and Hizbul-Islam are battling with government forces for control of the capital Mogadishu.
Somalia has been subject to a UN arms embargo for many years, but weapons are still freely available in the Mogadishu weapons market.