The flow of weapons into Sudan during this war has been analysed by various experts.
Amnesty International says it has found evidence of weapons manufactured in Serbia, Russia, China, Turkey, Yemen and UAE being used in Sudan.
The smuggling route is often via the UAE, through to Chad, then into Darfur - according to a leaked report by UN experts.
The UAE in particular is accused of providing arms and support to the RSF, who in turn are accused of using the UAE as a marketplace for illicit gold sales.
On Wednesday, Rubio pointedly said assistance to the RSF "isn't just coming from some country that's paying for it - it's also coming from countries that are allowing their territory to be used to ship it and transport it".
He also said he did not want to "diminish" the involvement of other actors in the conflict, saying "that includes potentially the Iranians, at least money and weapons being flown into the other side", meaning to the Sudanese army.
All parties deny these allegations.
A fortnight ago, the UK government came under fire from its own lawmakers following allegations that British-made weapons were ending up in the hands of the RSF, who were using them to commit atrocities.
In response to one MP's demand to "end all arms shipments to the UAE until it is proved that the UAE is not arming the RSF", Foreign Secretary Yvette Cooper said at the time: "The UK has extremely strong controls on arms exports, including to prevent any diversion. We will continue to take that immensely seriously."
There has been a UN arms embargo on the RSF's stronghold of Darfur since 2004, but it has not been extended to the rest of the country despite calls from human rights groups.