'Height of folly' found just to the right.
In a BBC interview, John Brennan also advised the new president to be wary of Russia's promises, blaming Moscow for much of the suffering in Syria.
In his campaign, Mr Trump threatened to scrap the Iran deal and also hinted at working more closely with Russia.
Mr Brennan will step down in January after four years leading the CIA.
In the first interview by a CIA director with the British media, John Brennan outlined a number of areas where he said the new administration needed to act with "prudence and discipline" - these included the language used regarding terrorism, relations with Russia, the Iran nuclear deal and the way in which the CIA's own covert capabilities were employed.
Mr Brennan offered a bleak assessment of the situation in Syria arguing that both the Syrian regime and the Russians were responsible for a slaughter of civilians which he described as "outrageous".
The administration of President Barack Obama has pursued a policy of supporting moderate rebels fighting the Assad regime in Syria. The CIA director said that he believed the US needed to continue that support to help rebels withstand what he called an "onslaught" carried out by Syria, Iran, Hezbollah and Russia.