Follow the latest on the earthquake in Turkey
The US on Thursday said Russia was largely to blame for complications in sending aid to Syria, throughout the conflict and especially after the devastating earthquake this week.
"Unfortunately, the reality has been up until now at least, that one country primarily has stood in the way," State Department spokesman Ned Price said.
It has become a contentious issue in the UN, where the Security Council has to regularly debate and approve aid access through Turkish-Syria border crossings — and Russia has threatened approval as a Syrian ally.
"There is really only one reason why there is only one border crossing point: That is because the Russian Federation has consistently vetoed or overturned any efforts to open additional border crossings.," Mr Price said.
"Now, in recent years, we have gone through this unnecessary exercise of reauthorising a single border crossing a vote in the UN Security Council.
"It has always been our contention that there should be a longer duration to this border crossing authorisation and that there should be more than only one border crossing."
Currently, Bab Al Hawa is the only route into the north-west region of Syria for aid deliveries to areas free of control by the government, because Damascus believes all assistance should go through it.
Many have accused the UN process of being "polticised" while so many Syrians are in need.
UN Secretary General Antonio Guterres hopes to expand aid operations in Syria with the addition of more border crossings.
The Syrian opposition and Turkey's government have agreed to add routes for earthquake response, a move not seen in years.
The US has also asserted that the Syrian government under President Bashar Al Assad has a role in limiting humanitarian work, as well as using such assistance for only government-held regions.
Inside World Health Organisation's Dubai warehouse packing emergency aid for Turkey and Syria
"We are calling on the Assad regime to immediately allow all of humanitarian aid in through all border crossings, and to allow humanitarian access to all persons in Syria in need, without any exception whatsoever," Mr Price said.
The Department has continued to try to combat beliefs that US sanctions are preventing aid work in Syria.
It has repeatedly posted on social media clarifications that its Syria policy or sanctions include exemptions for humanitarian efforts, and such posts have increased in number in relation to earthquake assistance this week.
"There are many hurdles to overcome when providing humanitarian assistance and Syria, and especially after devastating earthquakes this week, but our serious sanctions policy is not among them," Mr Price said.
The White Helmets, one of the few organisations working in north-west Syria responding to the earthquake situation, is supported by the US.
The US also says it is the largest provider of humanitarian aid to Syria, with at least $15 billion contributed since the war began in 2011.