Asked if Obama had made a beeline for him a the dinner or simply meandered in his direction, Mr Key said he would "put it more in the meandered camp. But he was very friendly."
He said he would talk to Obama again today and raise the Trans Pacific Partnership progress with him - free trade talks between nine Asia Pacific countries including the United States.
Mr Key also met with United Kingdom deputy prime minister Nick Clegg last night. He said Mr Clegg had briefed him on the economic climate in the United Kingdom and the recent Budget.
They had also had a broader discussion about Syria and Iran. He said Mr Clegg was grateful for the statements New Zealand had made about Syria and Mr Key had explained New Zealand could not take unilateral sanctions out against a country by law.
"That position could change but its certainly not something that's available to us at the moment."
It was the first time Mr Key has met Mr Clegg and the pair seemed to enjoy each other's company, laughing and joking about rugby as the meeting ended. Mr Clegg told Mr Key about a visit to New Zealand when a taxi driver was in despair because the All Blacks had only beaten the French by 40 to 12.
Mr Key also spent time talking to India Prime Minister Manmohan Singh and spoke to Russia President Dmitry Medvedev about Mr Key's upcoming trip to Moscow in June.
KEY TO CHALLENGE OTHER COUNTRIES TO REDUCE NUCLEAR STOCKS
Prime Minister John Key says he will use his four minute speaking slot at the Nuclear Security Summit today to challenge other countries to do more to reduce their stocks of nuclear materials and weapons.
Mr Key is in Korea for the summit - a meeting of the leaders of 54 different countries including the United States, China, India, Pakistan and Russia and among which New Zealand is the only completely nuclear-free country.
About 12 leaders presented their statements on nuclear security during the course of the opening dinner last night, which was closed to media.
Mr Key said leaders to speak included US President Barack Obama, who set out what the United States had achieved since 2010 and emphasised the ongoing importance and relevance of nuclear security.
Mr Key will present his today and said he would use the four minute opportunity to talk about the risks nuclear issues posed to New Zealand, despite it being a small country with anti-nuclear legislation.
"I'll challenge them to do a bit more, actually."
The Summit was instigated by President Obama in 2010 as part of his plan to deliver on a 2009 speech setting out his vision of a world free of nuclear weapons. The Summit's main aim is to ensure all nuclear material is either disposed of or safely secured within four years so it does not fall into the hands of terrorists. However, disarmament is always a subtext on the sidelines of the Summit.
This week President Obama pledged to continue with his goal - starting in his US homeland, despite the backlash from the Republican Party in the lead up to the United States elections later this year.
Obama told media after a speech in Seoul that the USA had more nuclear weapons than were necessary for its security interests and he intended to go further with the agreement he made with Russia in 2010 to reduce stocks.
The main session of the summit takes place today. Countries set out what commitments they will make toward increasing nuclear security, such as disposing of stocks of highly enriched uranium or plutonium and converting reactors to low-enriched uranium.
Several of the states which were left with old nuclear material after the breakup of the Soviet Union are also at the Summit.
New Zealand is expected to commit to ratifying two international conventions on nuclear security, including one that requires the secure use of radioactive material which is used for medical and industrial uses by all countries.
