The director of the CIA has warned US President-elect Donald Trump that ending the Iran nuclear deal would be "disastrous" and "the height of folly".
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.
'Beware Russian promises'
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 backing 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.
Russia continued to hold the key to Syria's future, he said, but he expressed scepticism about its willingness to come to any kind of deal. He said Moscow had been "disingenuous" in negotiating tactics, seeking to draw the process out in order to "choke" Aleppo.
"I do not have confidence that the Russians are going to relent until they are able to achieve as much tactical battlefield successes as possible," he said.
The incoming Trump administration has suggested it may try to work more closely with Russia on a number of issues.
"I think President Trump and the new administration need to be wary of Russian promises," Mr Brennan told the BBC, arguing Moscow had failed to deliver in the past.
On the role of Russia in trying to influence the US election by hacking and releasing information, the CIA director confirmed Russia had sought to carry out such activity but said he would defer to domestic counterparts as to the impact.
He did confirm that he had had conversations with his Russian opposite numbers to challenge them over these actions and warn them that they would backfire.
The US should not "stoop to their level" or risk escalation by responding in kind to Russian hacking but, he said, there were other ways of ensuring Russia understood such activity was unacceptable.
'Don't tear it up'
He also warned Donald Trump's incoming team over their position taken during the campaign to abandon the nuclear deal with Iran.
"I think it would be disastrous," Mr Brennan told the BBC. "First of all, for one administration to tear up an agreement that a previous administration made would be unprecedented."
He said such a move would risk strengthening hardliners in Iran and risk other states pursuing nuclear programmes in response to a renewed Iranian effort. "I think it would be the height of folly if the next administration were to tear up that agreement," he said.
In 2015 Iran agreed a long-term deal with six world powers - the US, UK, France, China, Russia and Germany - under which it agreed to restrict sensitive nuclear activities in exchange for the lifting of tough economic sanctions.
The Obama administration has argued that the deal will prevent Iran from obtaining a nuclear weapon but during his election campaign Mr Trump described it as a disaster and said he would dismantle it.
Since his election Mr Trump has not mentioned Iran but his choice to succeed Mr Brennan, Mike Pompeo, is an ardent critic of the deal.
IS 'very active'
Terrorism remains an overriding concern. The team planning external attacks within the so-called Islamic State remained "very active", Mr Brennan said, and was seeking to demonstrate that - despite setbacks on the battlefield - the group still had the ability to carry out attacks against the West.
The CIA director said he had not yet sat down with the new team to discuss the capabilities and programmes the CIA had but he was ready to do so.
"There are a lot of people out there who read the papers and listened to a news broadcasts where the facts may be a bit - you know - off. And so I want to make sure the new team understands what the reality is. It ultimately will be up to them to decide how to carry out their responsibilities."
Some members of the new administration, such as Gen Michael Flynn, have talked of the US needing to recognise it was in a "world war" with Islamist militants.
When asked if language about "world wars" was helpful, the CIA director said the new team needed to be "disciplined in the language that they use [and] the messages that they send. Because if they are not disciplined, their language will be exploited by the terrorist and extremist organisations as a way to portray the United States and the government as being anti-Islamic and we are not."
Mr Brennan said President Obama had asked US intelligence to "dig down" on whether the transition period might be exploited by adversaries.
The CIA is charged with gathering intelligence and acting as the covert arm of the president. Its activities are usually secret and often - when revealed - controversial.
One of the most public challenges that John Brennan faced during his tenure was dealing with the fallout of the CIA's use of techniques such as waterboarding on detainees after the 9/11 attacks.
President-elect Trump has said he would consider resuming waterboarding if he thought it would be effective. John Brennan made clear he thought that would be a mistake.
"Without a doubt the CIA really took some body blows as a result of its experiences," he said. "I think the overwhelming majority of CIA officers would not want to get back into that business."
The pace of drone strikes increased during the Obama administration although responsibility for many of them has shifted to the military rather than the CIA. When John Brennan ran counter-terrorism operations in the White House, he was instrumental in putting in place a series of rules over drone strikes.
So could a new administration simply rewrite the rules on drone strikes as well as the handling of detainees?
"This is where there is tremendous responsibility on the new administration…to make sure that they use the great capabilities that this government has as effectively and as judiciously as possible," he told the BBC, arguing that if powers such as drone strikes were misused, they could prove counterproductive to US security.