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A senior UN official's visit to the only border crossing for humanitarian aid into earthquake-struck northwest Syria on Sunday is "too little, too late", a rescue group has said.

UN aid chief Martin Griffiths visited the Bab al-Hawa crossing, where he met White Helmets head Raed al-Saleh, according to a press release from the Syria Campaign advocacy group.

Al-Saleh met Griffiths at the crossing between Turkey and Syria's Idlib province to "demand" more cross-border routes are opened without waiting for UN Security Council authorisation, the press release said.

"Since the moment the earthquake struck seven days ago we have been appealing to the UN to send urgent assistance to aid our rescue operations in northwest Syria," al-Saleh said.

"For days these calls went unheeded and during this time countless lives have been needlessly lost. This visit is too little, too late.

"The UN's failure to act quickly to save Syrian lives in the face of a humanitarian catastrophe is utterly shameful and should be a stain on its conscience."

He said waiting for approval from the Security Council to reopen additional border crossings into Syria's northwest is "completely misguided", adding "this unnecessary stalling will only cost more lives". He said there "can be no more delays".

"We urgently need the UN to open more border crossings into northwest Syria so that cross-border humanitarian aid can flow in unhindered," al-Saleh said.

"Failing to escalate medical aid deliveries rapidly will leave the UN with more blood on its hands."

Monday's earthquake has so far killed more than 3,500 people in Syria and over 24,600 in neighbouring Turkey.

Griffiths, the UN aid chief, tweeted on Sunday: "At the [Turkey-Syria] border today. We have so far failed the people in north-west Syria.

"They rightly feel abandoned. Looking for international help that hasn't arrived.

"My duty and our obligation is to correct this failure as fast as we can. That's my focus now."

Griffiths' visit comes as a UN spokesperson told Reuters on Sunday that earthquake aid from regime-held parts of Syria into territory controlled by hardline opposition groups has been held up by approval issues with Islamists Hayat Tahrir Al-Sham (HTS).

Extremist group HTS controls vast swathes of Idlib province and surrounding areas in the country's northwest.

An aid convoy from the UN and another from a Turkish humanitarian group arrived at Bab al-Hawa on Saturday, the crossing's media relations director Mazen Alloush told °®Âþµº's Arabic sister site Al-Araby Al-Jadeed.

The Syria Campaign has started a addressed to UN Secretary-General Antonio Guterres, Griffiths and the world's governments. More than 11,600 people have signed it so far.

"You must immediately direct all governments and international aid organisations to deliver the heavy machinery, technical support, and humanitarian aid that northwest Syria needs in response to the earthquake," the petition reads.

"All border crossings must be utilised without delay."

Reuters contributed to this report.