Al-Shabaab remains “a determined, ruthless enemy with quite a lot of capability” despite its recent loss of territory, a top UN envoy for Somalia has said.

According to Mr Nicholas Kay, attacks by the Islamist insurgents could even force UN personnel to withdraw from the country.

"If we make a mistake in our security presence and posture, and suffer a significant attack, particularly on the UN, this is likely to mean to us withdrawing from Somalia," he told a meeting in Washington.

Al-Shabaab also remains intent on striking in neighbouring countries, including Kenya, he added.

The militants have killed two members of the Somalia Parliament in the past two days.

READ: Second Somali lawmaker killed in Mogadishu

Two UN consultants were shot dead earlier this month, and more than 20 people killed during an attack on the UN compound in Mogadishu last June.

An offensive launched last month against the militants by forces from the African Union peacekeeping (Amisom) and the Somali government pushed Al-Shabaab fighters out of about 10 towns, Mr Kay noted.

The group has consequently been deprived of revenue sources and training bases, the UN diplomat said.

A second phase of the offensive is expected to begin “quite soon,” although Amisom and Somali troops are facing supply challenges due to Al-Shabaab's continued control of much of the countryside, Mr Kay added.

Amisom lacks the ability to protect supply routes from the air because it has not been supplied with helicopters, the UN envoy noted.

“It is disturbing to me that no State has come forward to supply helicopters to the AU mission,” he said.

“There is progress,” Mr Kay said in summary, “and that progress is in peril.”