THE PENTAGON -- President Obama said Thursday that Donald Trump's claim that November's election will be rigged is "ridiculous" and he also warned that making "bad decisions" like imposing a religious test will backfire in the fight to defeat the Islamic State in Iraq and Syria (ISIS).

The president spoke to reporters at the Pentagon after meeting with his National Security Council (NSC) for several hours for an update on the ISIS fight. During the press conference, he was asked to react to Trump's claim that the election will be rigged.

Obama on ISIS analysis

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"I don't even really know where to start. Of course the elections will not be rigged. What does that mean?" Mr. Obama said.

He added that it's crazy that Trump is suggesting there's a conspiracy theory being propagated across the country, including in places like Texas, which is largely controlled by Republicans.

"That's ridiculous. That doesn't make any sense and I don't think anybody takes that seriously," Mr. Obama said. "My suggestion would be you know, go out there and try to win the election. If Mr. Trump is up 10 or 15 points on Election Day and ends up losing, then maybe he can raise some questions. That doesn't seem to be the case at the moment."

The president talked about the progress that has been made in the fight to defeat ISIS overseas and the challenges that remain to prevent terror attacks abroad and at home. Mr. Obama said that ISIS "continues to lose territory on the ground" in Iraq and Syria after more than 14,000 coalition airstrikes and the coalition continues to take out senior ISIS leaders.

"None of [ISIS's] leaders are safe and we're going to keep going after them," said Mr. Obama, who said that the U.S. has gained vast amounts of intelligence as it has beaten back the terror group in Iraq and Syria.

Asked if he's satisfied with the U.S. response, he said, "I'm never satisfied with our response because if you're satisfied that means the problem's solved and it's not."

He acknowledged that it's tougher to root out networks that plan more complicated attacks and he said while networks are more active in Europe, it's "conceivable that there are some networks here that could be activated."

The president said Americans should "not panic, not succumb to fear" and that the U.S. is still strong.

"[ISIS] cannot defeat the United States of America or our NATO partners. We can defeat ourselves though if we make bad decisions," he said.

Obama ISIS press conference

Mr. Obama said that those bad decisions include indiscriminately killing civilians or imposing an offensive religious test on people -- a clear swipe at Trump and his supporters.

"Those kinds of strategies can end up backfiring. We cannot frame this as a clash of civilizations between the West and Islam."

Asked what he thinks about Trump receiving classified intelligence briefings now that he is the Republican presidential nominee, Mr. Obama said his administration will follow the law and still provide them so that if they win, "they're not starting from scratch so that they are prepared for this office."

"What I will say is that they have been told that these are classified briefings. If they want to be president, they've got to start acting like president," he said.

The president shot down a report by The Wall Street Journal a day earlier that the U.S. had sent $400 million in cash to Iran as American detainees were freed as a ransom payment.

"We announced these payments in January, many months ago. It wasn't a secret. We announced them to all of you. Josh did a briefing on them," Mr. Obama said Thursday.

"We do not pay ransom for hostages," added the president, who explained that the only news is that the U.S. sent the money through cash instead of a check or wiring the funds because the U.S. "doesn't have a bank relationship with Iran."

The president also slammed Russia for its involvement in Syria recently, which he said "raises serious questions" about its commitment to a cessation of hostilities, but he said the U.S. is prepared to work with the Russian government in terms of the Syrian civil war.

And Mr. Obama blasted Congress for not taking action to pass his funding request for efforts to fight the Zika virus. The U.S. is now seeing cases of the Zika infection in Florida that were transmitted through mosquitoes on the mainland.

"Congress needs to do its job. Fighting Zika costs money."