Denmark, already a big donor, said it could help the UN programme, which was supporting 1.7 million Syrian refugees, but it blasted Gulf states for not paying up.
"At the large conference in Kuwait in January this year, Denmark pledged 200 million kroner (26.9 million euros, $33 million) to the UN, and all of that has been paid," Danish Minister for Trade and Development Mogens Jensen told AFP.
"Countries in the region such as Saudi Arabia and Kuwait have not yet fulfilled their promises," he added.
The Danish minister on Thursday told daily Politiken that "maximum pressure" should be put on the "rich" Gulf states to honour "lofty" promises made at a Syria donor conference in Kuwait on January 15.
Kuwait has paid 60 percent of the $500 million donation announced at the conference, while Saudi Arabia has paid 28 percent of the $60 million it said it would give, according to the UN Office for the Coordination of Humanitarian Affairs (OCHA).
Saudi Arabia and Kuwait had a "regional interest" in securing a more stable Syria, Jensen said.
The UN's World Food Programme (WFP) launched a social media campaign on Wednesday to raise $64 million needed to help feed nearly 1.7 million Syrian refugees after donors failed to honour cash pledges.
It was forced to make the plea after running out of funds to pay for its food voucher programme for refugees in Jordan, Lebanon, Turkey and Egypt.
However, most of the money pledged at the donor conference in Kuwait was not earmarked for the WFP and could go to other humanitarian work in and around Syria.
The leader of the moderate Syrian opposition said on Friday
that cuts to UN food aid for 1.7 million Syrian refugees amounted to "an execution order" overseen by the international community.
"This amounts to an execution order for killing 1.7 million people under famine, especially during this harsh wintertime," Syrian National Coalition President Hadi al-Bahra said at a press conference in Copenhagen.