KABUL - Afghanistan faces the risk of a new spiral in violence following a series of attacks on civilians in the last few months but on the battlefield, security forces have been holding their ground, the top UN official in the country said.

The assessment by Nicholas Haysom, the UN Secretary General's Special Representative for Afghanistan, painted a mixed picture of the security situation some 18 months after international forces ended their main combat operations.

While the bomb attacks that killed at least 22 people in Kabul and the northern province of Badakhshan on Monday underlined how dangerous Afghanistan remains, fears that the Taliban would overwhelm security forces have not been realised.

However Haysom said he was deeply concerned about violence against civilians, with attacks continuing during Ramadan, Islam's holy month.

As well as suicide attacks on targets ranging from government officials and members of the judicial system to foreign security contractors, at least 200 people have been abducted on highways since the end of May.

"There is a risk, in my view, that the conflict may enter a new phase, which could see retaliatory acts of vengeance and an escalating spiral of violence," said Haysom, who is stepping down from his post after four years in Afghanistan.

Meanwhile, the Shanghai Cooperation Organisation (SCO) will discuss a "very tense situation" in next-door Afghanistan when the regional body led by China and Russia holds its summit in Uzbekistan this week, Kremlin aide Yuri Ushakov said on Wednesday.

An estimated 60,000 armed rebels are now fighting in Afghanistan, including 10,000 under the flag of Islamic State, Ushakov told journalists.

The security and cooperation body, which also includes post-Soviet Central Asian nations of Kazakhstan, Kyrgyzstan, Tajikistan and Uzbekistan, is expected to accept Iran as its seventh fully fledged member, Ushakov said.

"Our position is crystal-clear: Iran sent its membership bid as early as April 2008 and now that sanctions imposed on Iran have been lifted, Russia believes there is no reason to refrain from accepting this country to the organisation," he said.

Regional heavyweights India and Pakistan are expected to become SCO members at the next summit to be held in Kazakhstan in 2017, Ushakov said.