The US insists that all of its aid to the Presidential Guard is "nonlethal," consisting of training, uniforms, and supplies, as well as paying for better infrastructure at Gaza's borders.
Regional analysts and Palestinian officials say the rifles being provided to the guards are being provided by other Arab states with close ties to the US.
And supporters of the program say the US has little choice but to back Abbas as the best hope for Israeli-Palestinian peace.
"As a soldier, I believe there's a point when inaction, a wait-and-see attitude, is no longer an option," said Lt. Gen. Keith Dayton, who is overseeing the program, in testimony to a congressional subcommittee Wednesday.
"The situation has gotten to be quite dire in Gaza, we have a situation of lawlessness and outright chaos," he said. "This chaotic situation is why the [US] is focused on [helping] the legal, legitimate security forces in our effort to reestablish law and order."
Abbas now finds himself engaged in an elaborate, multiparty dance involving Hamas, the US, and the Israelis.
He has been quietly urging Palestinian Prime Minister Ismail Haniyah, of Hamas, to stop rocket fire at Israel by the Islamic militants, something that has won him praise from the Americans and Israelis. But gunmen from his Fatah movement have also engaged in a series of battles with Hamas militants in recent weeks, killing at least 50. The two sides have reached a tenuous truce.
Last week, when that fighting veered towards open warfare between the Palestinian factions, Israel allowed about 500 Fatah loyalists to cross back over the Rafah crossing into Gaza from Egypt, where they were receiving US training, an unusual move for Israel, which seeks to strictly limit the movement of fighting-age men through the Gaza border with Egypt.