In fact the USA ended all support for the rebels a while back after they saw that weapons were being sold or lost by rebel groups to more extremist groups.

The Obama admin and its allies support for Salafi jihadists was a deliberate policy, it was no accident.

What actually happened was that the US was fully aware even in 2012 that almost every weapon it was sending to Syria was ending up in the hands of the jihadists. The FSA routinely worked alongside al Nusra and gave or sold large amounts of their US supplied weapons to them. By 2013 the "moderate" FSA (which was never moderate) had collapsed and the majority of its fighters joined the Islamists or fled to Turkey.

After Defense Intelligence Agency's classified 2013 report assessed that the removal of the Syrian government would likely lead to country being run by disparate jihadi groups the Obama admin had a decision to make.

On the one hand continuing the arms smuggling into Syria would be handing the country to Salafi jihadists but on the other hand doing so might remove Syria as a Russian and Iranian ally.

The Obama admin against the advice of the joint chiefs decided that the gain of removing Assad was worth the cost of handing the country to jihadists.

This caused a massive conflict inside the admin and the joint chiefs eventually began deliberately undermining the admin's support for the jihadists. While one hand of the Obama admin was arming the jihadists the joint chiefs were sending US military intelligence through the Germans to the Syrian Army.

‘The Joint Chiefs believed that Assad should not be replaced by fundamentalists. The administration’s policy was contradictory. They wanted Assad to go but the opposition was dominated by extremists. So who was going to replace him? To say Assad’s got to go is fine, but if you follow that through – therefore anyone is better. It’s the “anybody else is better” issue that the JCS had with Obama’s policy.’ The Joint Chiefs felt that a direct challenge to Obama’s policy would have ‘had a zero chance of success’. So in the autumn of 2013 they decided to take steps against the extremists without going through political channels, by providing US intelligence to the militaries of other nations, on the understanding that it would be passed on to the Syrian army and used against the common enemy, Jabhat al-Nusra and Islamic State.