In an interview published on Tuesday, Secretary-General Stoltenberg said the alliance will respond to any attack on Poland or the Baltic countries.
Before heading to London, Erdogan suggested that Turkey might not back Poland and NATO's Baltic allies - Estonia, Latvia and Lithuania - should they require defending unless the allies support Turkish concerns about Syrian Kurdish fighters, which Ankara sees as terrorists.
The President, along with other world leaders, will be hosted by the Queen at Buckingham Palace later Tuesday evening.
I said that we could do that with pleasure.
The senior government official would not say whether the USA sent a letter ahead of this week's summit in London.
"The question is, as we celebrate 70 years, are we waving in celebration or do people think we are drowning?" said a senior European NATO diplomat. But even the British hosts, for generations the most enthusiastic champions of the trans-Atlantic partnership that NATO represents, are disunited over their project of quitting the European Union and distracted by a rancorous election next week.
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"We are faced with a paradox", Stoltenberg told Reuters before the summit.
A White House statement last month said: "President Trump looks forward to meeting with the other NATO Heads of State and government to review the Alliance's unprecedented progress on burden-sharing, including adding more than $100bn in new defence spending since 2016".
Differences among the leaders have emerged over funding and strategic decisions.
"All NATO countries are obliged to support the reform of the alliance, this will allow a more principled and decisive fight against terrorist organisations", he added.
Turkey wants North Atlantic Treaty Organisation to formally recognize the YPG militia, the main component of the USA -backed Syrian Democratic Forces (SDF), as terrorists and is infuriated that its allies have given the militia support.
A Franco-German proposal would create a group of eminent figures to consider the alliance's future political role, submitting a report by the next scheduled summit in late 2021. While that is more than several years ago, it still ranks in the bottom half of alliance members, at 20th out of 29 countries.