The former Labour Chancellor has been invited by the International Monetary Fund (IMF) to take part in its spring talks, and will speak in a seminar on global finance and economic recovery on Tuesday.
However, the British Embassy in Washington is also arranging meetings for him, outside the IMF, so he can talk about Scotland.
It is understood Darling hopes to discuss the referendum with some of Washington's powerful political and economic think tanks.
Better Together last night confirmed Darling would take part in a range of referendum-related engagements in Washington, although his schedule was still being finalised.
The UK Government also insisted it was entirely appropriate for the Foreign and Commonwealth Office (FCO) to help Darling in this role.
But the SNP attacked the "abuse" of UK diplomatic resources.
It is standard protocol for Westminster and Scottish ministers to receive local embassy support during foreign visits. However, Darling is not a minister, but the head of a political campaign.
Although he is not allowed to use embassy premises for Better Together meetings, the FCO has agreed embassy staff can facilitate talks elsewhere and help organise his schedule.
Westminster's justification for helping Darling is that Better Together advocates the UK Government policy of keeping the Union.
The SNP said it was reasonable for ministers to receive embassy support to promote government policy, but wrong for Darling to have the same treatment while a campaign leader.
SNP Westminster leader Angus Robertson said: "This is an abuse of UK diplomatic resources by the No campaign. The UK Embassy in the US appears to be arranging meetings and interviews for Alistair Darling in his capacity as head of a political campaign, which steps way over the line.
"Mr Darling complains about public money to promote awareness of the White Paper so that people are informed about the referendum. The No campaign has laid itself wide open to the charge of hypocrisy."
Darling's visit coincides with Scotland Week in North American and Canada, where First Minister Alex Salmond will be advocating Scottish independence.
In a speech tomorrow he will say Scotland could be a great nation, if not a great power.
Giving the inaugural Glasgow Caledonian lecture in New York, he will say: "Countries can exercise influence through the scale of their ambition and the strength of their ideas, rather than the size of their armies, their populations, or their territories. Scotland can be a progressive beacon, setting a positive example as a country which combines fairness and prosperity."
Salmond said yesterday he expected the US Government, which is wary of the UK breaking up with the loss of the Trident nuclear deterrent, to keep out of the referendum. He said: "I don't foresee pressure. I don't think that is what the United States would want to do."
Better Together claimed Darling was receiving Embassy support because he was an Edinburgh MP.
A spokesperson said: "It's laughable for the nationalists to complain about this given that Alex Salmond will be in the States at the same time lavishly spending public money."
A UK Government spokeswoman said: "It was entirely appropriate for the FCO to provide assistance to the former chancellor during his visit to the United States, which included meetings with the IMF.
"UK embassies provide considerable support for Scottish Government Ministers overseas, including roughly 50 visits in the last 16 months, and facilitation of [SNP External Affairs minister] Humza Yousaf's programme of visits to San Francisco and Vancouver as part of the Tartan Week celebrations."