US-led attacks on the jihadis of the Islamic State (Isis) are the product of a confused policy that is “turning a blind eye” to the crimes of President Bashar al-Assad, according to the leader of Syria’s main western-backed opposition group

Hadi al-Bahra, president of the Syrian National Coalition (SNC), warned in an interview on Monday that air strikes against Isis by the international coalition assembled by Barack Obama were weakening support inside Syria for already embattled non-extremist anti-Assad forces.

“The coalition is fighting the symptom of the problem, which is Isis, without addressing the main cause, which is the regime,” Bahra told the Guardian before a meeting of the 11-nation Friends of Syria forum in London. “People see coalition planes hitting Isis targets but turning a blind eye to Assad’s air force, which is using barrel bombs and rockets against civilian targets in Aleppo and elsewhere.”

Assad’s office said on Monday that a UN proposal for a freeze on fighting in Aleppo, which is divided into rebel- and government-held areas, was “worth studying”.

However, Bahra said local ceasefires would only benefit the regime unless they were part of a comprehensive, negotiated political solution to a conflict that has cost 200,000 Syrian lives and displaced half the population since March 2011.

However, his main message was about the damage to the anti-Assad cause since the US and four Arab allies began targeting Isis in Syria in late September in tandem with parallel attacks across the border in Iraq. “People feel there is a hidden agenda and cooperation between the coalition and Assad’s forces because Assad assumes he has a free hand,” he said. “Syrian public opinion is a front which we need to win.”

Hitting targets other than Isis – such as the al-Qaida-affiliated Jabhat al-Nusra – was also playing into the hands of the Damascus government, he said.

Bahra decried the fact that the US-led coalition was not liaising with the fighters of the Free Syrian Army, a loose alliance of anti-Assad units linked to the SNC. The only exception has been at Kobani on the Turkish border, where Kurdish and FSA forces worked in coordination with coalition air strikes to push back Isis.

“The FSA is being ignored completely and this is weakening the international coalition operation because it is not able to achieve results on the ground,” he said. “The whole operation has been confused. Air strikes will not be able to win the battle against extremism. You have to defeat Isis on the ground. And you have to deal with the main cause and source of extremism, which is the regime itself.”

Bahra played down recent defeats for two US-backed rebel groups in the Idlib area and insisted there were plenty of other nationalist fighting forces available to take part in a planned $500m (£315m) “train and equip” programme. But he admitted there had been many negative aspects to opposition military strategy.

“Isis works under one command and with a clear ideology,” he said. “Aid to the FSA was delivered to battalion commanders so fighters got sick and tired of belonging to an individual and not to a united national force which works to achieve the aspirations of the Syrian people.”

Bahra also made clear his opposition to ceasefire proposals being explored by the UN envoy to Syria, Staffan de Mistura, which have been discussed in recent days in a flurry of unofficial reports and leaks.

“The lives of Syrians are our top priority but we need to look at this from a strategic point of view,” the opposition leader said. “Ceasefires and limiting existing violence provide a temporary solution but not a permanent resolution of the crisis. Ceasefires without a clear vision for a full and comprehensive political solution will give the regime time to regroup and reorganise itself to continue its crimes against the Syrian people at a later stage.”

At the meeting in London, Bahra met the foreign secretary, Philip Hammond, who said afterwards that Assad’s regime was “unwilling and unable to take effective action” against Isis. Hammond added: “I reaffirmed today to President al-Bahra that Assad can play no future role in Syria.”

Bahra, a US-educated engineer with close ties to Saudi Arabia, has battled to improve the image of the SNC, scarred by infighting and manipulation by Turkey and Gulf sponsors. Activists on the ground routinely attack it for being out of touch, unrepresentative and ineffective. Last July, Jon Wilks, the UK government envoy to the opposition, warned privately that if the body behaved like a “mafia” it would adversely affect donor funding.

Its position has worsened recently because the Isis security threat has led some western governments to consider reopening channels to Assad and dealing with him as the “lesser evil” to the jihadis. “At this point it doesn’t look good,” Bahra admitted. “But sooner or later the international community will have to find a permanent solution.”

Bahra headed the opposition delegation to the Geneva II talks last January, which petered out without any progress and were followed by Assad’s re-election for a third term as president. Bahra said he saw little prospect for a return to negotiations.

The allies of the Syrian opposition, especially the US, had been generous with humanitarian aid but had provided far less financial and military support than Iran and Russia had given Assad, Bahra said. “Our friends are not serious enough in providing the proper amount of aid and support to help us to create enough military pressure inside Syria to press the regime to return to the negotiating table to achieve a full political solution.”