US National Security Adviser John Bolton (Picture: PA).
One of Donald Trump's closest advisers says the UK's departure from the EU will boost NATO, giving ministers a more powerful international voice.
John Bolton, the US national security adviser, is visiting London.
Mr Bolton has told reporters he has also been assured the UK will re-examine whether to allow Chinese tech firm Huawei to work on 5G mobile networks.
Having met with Boris Johnson, Mr Bolton said the UK will be "first in line" for a trade deal with the US - which could pursue a "sector-by-sector" deal with Britain.
The security official also said issues like security in Iran, and fears over Huawei's involvement in the UK's 5G network could wait until after Brexit is resolved.
He said: "The message I wanted to convey on Iran, and on some other issues in which I include China, 5G, Huawei, that cluster of issues, is that the President and the US Government fully understands that in the next few days the UK Government has a singular focus on the Brexit issue, so that we are not hoping for anything on these broad and complex questions.
"We just ask that, as issues come up, we resolve them individually and we reserve the time to have a larger conversation on some of these important issues at a moment that is really right for the new government. We just felt we owe them that.
"Obviously we have views on these issues, I think that is appreciated by the new government. They said in particular that looking really from square one on the Huawei issue that they were very concerned about not having any compromise in the security of telecommunications in the 5G space."
"We don't want to put you under pressure on these issues. There will be time enough to talk, that is really all we ask for."
Mr Bolton said he is "pleased" that Boris Johnson's government had agreed to participate in Operation Sentinel to improve security in the Persian Gulf, saying this "reflects a change from the prior government".