Washington — President Barack Obama vigorously denied on Thursday that a $400m cash payment to Iran was ransom to secure the release of four Americans jailed in Tehran. He defended the transaction as evidence that the nuclear accord with Iran has allowed for progress on other matters."This wasn't some nefarious deal," Obama said during a news conference at the Pentagon.The money was delivered to the Iranian government in January, at the same time the nuclear deal was settled and the Americans were released. The payment was part of a decades-old dispute over a failed military equipment deal dating to the 1970s, before the Islamic revolution in 1979.He pointed out that the payment, along with an additional $13bn in interest to be paid later, was announced by the administration when it was concluded in January, a day after the implementation of a landmark nuclear agreement with Iran. "It wasn't a secret. We were completely open about it," he said.Obama allowed that the one piece of new information, first reported this week by The Wall Street Journal, was that the $400m was paid in cash. It was delivered to Iran on palettes aboard an unmarked plane."The only bit of news is that we paid cash," he said. "The reason is because we couldn't send them a check and we couldn't wire the money. We don't have a banking relationship with Iran which is part of the pressure we applied on them."Classified briefingsObama also answered political questions at the news conference, pushing back at Republican nominee Donald Trump's suggestions that the November election might be rigged, calling the assertion "ridiculous". He said his advice to Trump, a candidate he has declared "unfit" for the presidency, was to "go out there and try to win the election".Trump and Democratic nominee Hillary Clinton will soon be receiving classified briefings, where they will gain access to sensitive information about national security and America's military posture. Asked whether he was worried about Trump having access to such material, Obama said simply that those who want to be president need to start acting like it."That means being able to receive these briefings and not spread them around," he said.The president's appearance before reporters followed an hours-long meeting with military leaders at the Pentagon on the fight against the Islamic State group.Obama said there have been gains in weakening ISIS in Iraq and Syria, but he conceded the extremist group still poses a threat to the United States as it shifts its tactics to carrying out attacks elsewhere around the world. While those attacks may result in less carnage, Obama said ISIS knows they still create "the kinds of fear and concern that elevates their profile".The rise of the Islamic State has kept Obama tied to the Middle East in a way he had hoped to avoid in his eighth and final year in office. While the US has far fewer troops in the region than when he took office in 2009, Republicans argue that the drawdown of troops from Iraq created a vacuum that allowed the Islamic State to thrive.Asked whether he feels any personal disappointment about not being able to do more to stop the Islamic State, Obama said "I haven't gotten numb to it. It bugs me."On Syria, the president criticised Russia's support of government attacks against opposition forces and its sieges of cities such as Aleppo. He accused Russia of failing to take steps to reduce violence in Syria — where a civil war has raged for much of Obama's presidency — but said the US would continue trying to push Moscow to focus on the fight against ISIS and other extremists.The president's session at the Pentagon occurred as the US was bombing targets in and around the Libyan city of Sirte, a notable expansion of the US-led coalition's military mission against ISIS. At the urging of the Pentagon, Obama authorised the strikes that started this week and include precision attacks against ISIS tanks, rocket launchers and fighting positions.Mired in chaos following the ouster of strongman Moammar Gadhafi in 2011, Libya became a target for ISIS extremists hoping to build a safe haven outside its initial territory in Iraq and Syria. Though the number of ISIS fighters in Libya has dwindled, the US is hoping to help Libya's fledgling UN-backed unity government finish the job.