Jeremy Corbyn will not seek Britain's exit from Nato or the European Union but will maintain opposition to the renewal of Trident, a left-wing ally said as the new Labour leader began picking a shadow cabinet.
Diane Abbott said the issues were "red herrings" after deputy leader Tom Watson conceded there were policy differences on key foreign policy issues that would require compromises if the party was to unite behind the new regime.
Mr Corbyn was declared Ed Miliband's successor on Saturday on the back of a surge of support from activists that saw him attract a massive 59.5% of votes - topping the ballot among party members as well as trade unionists and new supporters.
The immediate resignations of a string of senior figures who declared themselves unable to serve in his top team underlined, however, the task he faced uniting the party behind his anti-war and anti-austerity platform.
Prime Minister David Cameron - who last night telephoned his new opponent to congratulate him - issued a stark warning that Labour under him posed "a threat to our national security, our economic security and your family's security".
Mr Watson - appearing on BBC1's Andrew Marr Show after Mr Corbyn pulled out of a planned interview - said he understood the concerns of moderate colleagues about the dramatic political shift but urged them to "respect" the mandate given by voters.
He insisted there was "zero chance" of a successful coup against the new leader who was seeking "a front bench that represents all the talents and all the views".
In a signal of the potentially divisive battles to come, Mr Watson - seen as a crucial linchpin in securing party unity - accepted that he and many other MPs opposed Mr Corbyn over the renewal of the nuclear deterrent.
He conceded he didn't know "precisely what Jeremy's position" was on Nato membership - which Mr Corbyn has previously opposed - and hoped to "convince him of the merits".
And he insisted the party could present a united front in favour of European Union membership - despite Mr Corbyn failing during the contest to make clear whether he would campaign to stay in at the forthcoming referendum.
"These things have got to be worked out," he said.
Ms Abbott told BBC Radio 4's World This Weekend: "I think they should stop reading the papers about what Jeremy thinks and ask Jeremy what he thinks.
I can say with confidence Jeremy is not taking us out of Nato and he is not taking us out of the EU, those are red herrings.
"If they have some passionate commitment in principle to nuclear weapons there may be a difficulty for them with a Jeremy Corbyn leadership.
"But if they have a passionate commitment to nuclear weapons they'll find themselves out of step to the new Labour Party."
Mr Corbyn, who spent part of the morning at a community "fun day" in his constituency, was later pictured signing confirmation of his leadership for the Electoral Commission before starting work on the new front bench line-up.