WASHINGTON (CNN) -- Sen. John McCain warned Tuesday that Iran's increasing influence in the Middle East is hindering progress in Iraq.

Sen. John McCain discusses Iran's influence on the Middle East on Tuesday.

Ending a weeklong trip to the region with a congressional delegation that included a stop in Iraq, McCain expressed concern over a large cache of explosives found in Iraq and hinted that they may have been sent from Iran.

During a press conference in Amman, Jordan, the Arizona senator also said that there are continued fears that Iran may be training Iraqi extremists in Iran and then sending them back into Iraq.

The presumptive Republican presidential nominee said he was concerned about Iran developing nuclear weapons and said he planned to work closely with European allies to create strict sanctions "that would be harmful and compelling" to Iran's trade, diplomatic and financial institutions if he is elected president. Watch McCain discuss Iran and Iraq »

"There'd be a broad range of sanctions and punishments to the Iranians to help try to convince them that their activities -- particularly development of nuclear weapons -- is not a beneficial goal to seek," he said.

McCain also made a gaffe Tuesday when he said several times that Iran, which is mostly a Shiite country, was training the predominantly Sunni militant group al Qaeda.

Sen. Joe Lieberman, I-Connecticut, whispered in his ear. Moments later, McCain responded, "I'm sorry, the Iranians are training extremists, not al Qaeda."

McCain, who is known for having a tough stance on foreign policy, came under enormous scrutiny last year at a campaign event in South Carolina when he joked about bombing Iran.

Singing a rendition of the Beach Boys song "Barbara Ann," McCain changed the lyrics to "Bomb Iran," singing, "bomb, bomb, bomb, bomb, bomb Iran."

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Throughout his presidential campaign, McCain has stood firm on his support of the war in Iraq, even though he acknowledges that it could cost him the Oval Office.

McCain said last month that if he can't convince war-weary Americans that the U.S. policy in Iraq is succeeding, he will lose.

"Let me just put it this way: Americans will judge my candidacy first and foremost on how they believe I can lead the country both from our economy and for national security. Obviously, Iraq will play a role in their judgment of my ability to handle national security," McCain said.

Wednesday marks five years of U.S. involvement in Iraq. Democratic presidential hopefuls Sens. Barack Obama and Hillary Clinton have highlighted stark differences with McCain and have urged a speedy withdrawal.

But those differences are not having a large impact on whom voters will choose in November, according to a poll released Monday.

In a CNN/Opinion Research Corporation Poll, Obama and Clinton would both statistically tie with McCain in the November matchup.

If Obama were to win the nomination, he would garner 47 percent of the vote, compared with 46 percent for McCain, a statistical tie given the poll's 3 percentage point margin of error.

Should Clinton win the nomination, the poll indicates that she would get 49 percent, compared with McCain's 47 percent, another statistical tie.

Nonetheless, McCain said he was encouraged by the progress he saw in Iraq as well as in his meeting with King Abdullah II of Jordan.

"We realize that there are enormous challenges in the form of the Israeli-Palestinian peace process and also continued efforts to win the struggle in Iraq, which we are succeeding, but we still have a long way to go," he said.

Despite having a fundraiser planned in London on Thursday, McCain stressed that the trip, his eighth to Iraq, was not political. Watch more on McCain's trip to Iraq »

McCain is the ranking Republican member of the Senate Armed Services Committee. He was joined on his trip by two fellow committee members, Lieberman and Sen. Lindsey Graham, R-South Carolina. E-mail to a friend

CNN's Ed Hornick contributed to this report.

All About John McCain • U.S. Presidential Election • Iran • Iraq