WASHINGTON -- Republican front-runner Donald Trump vowed Wednesday to put U.S. security "above all else" if he is elected president, warning allies that they would be left to defend themselves if they don't "pay their fair share."

He also called for an easing of tensions with Russia "from a position of strength, only."

"'America first' will be the major and overriding theme of my administration," Trump said in a 38-minute address at the Mayflower Hotel in Washington.

Trump -- fresh off a sweep of five Northeastern primaries that pushed him closer to the GOP presidential nomination -- sought to expand on his foreign-policy views that have lacked detail and worried experts in both parties. But he left some questions about his proposals unanswered, including how he would defeat the Islamic State group, casting his sparse details as part of a strategy to catch U.S. enemies off guard.

"We must, as a nation, be more unpredictable," Trump said. He read his remarks from a teleprompter, a notable change for a candidate who typically speaks off the cuff.

Trump appeared to send a mixed message to U.S. allies. He doubled down on his previous assertion that other countries must contribute to international security agreements, such as NATO, if they're to get the benefits of American military protection. Yet, he also assured allies that the U.S. will have a renewed commitment to its overseas friends if he's elected.

"To our friends and allies, I say America is going to be strong again. America is going to be reliable again," Trump said. "It's going to be a great and reliable ally again."

He called for a summit with NATO allies and Asian partners shortly after he takes office. And he made direct appeals to Russia and China, saying the U.S. and those world powers are "not bound to be adversaries."

"We should seek common ground based on mutual interests," Trump said, citing Russia's concerns about the rise of Islamic extremists.

Many elements of Trump's speech were similar to his typical campaign remarks, but he delivered them in a more sober, restrained manner. Critics have said repeatedly that he has not shown an ability to act and sound "presidential."

"This speech is the most dramatic evidence thus far that Donald Trump fails the presidential test," said Texas' U.S. Sen. Ted Cruz, Trump's chief rival in the race for the GOP nomination. Cruz noted that Trump's convention manager was "widely recognized for his entanglements with corrupt foreign regimes and anti-democracy rulers."

Trump's speech did not appear to change other critics' view about his preparedness for being commander in chief. Sen. Lindsey Graham, R-S.C., said on Twitter that Trump's speech was "pathetic in terms of understanding the role America plays in the world.

"Final thought on Trump's foreign policy speech -- Ronald Reagan must be rolling over in his grave," Graham wrote.

But Republican former presidential candidate Newt Gingrich voiced support.

"This was a serious foreign policy speech by Trump," he tweeted. "It is worth reading and thinking about. It will be ridiculed by Washington elites."

The chairman of the Senate Foreign Relations Committee, Bob Corker of Tennessee, called it "a very good foreign-policy speech," but said he hoped for a time when "candidates in both parties will begin focusing not only on the problems we face but on solutions."

Jim Gilmore, a former Virginia governor and 2016 presidential candidate who attended the speech, said he didn't hear any new policies, describing it as a mix of being "willing to pull back" from foreign engagements and "strong belligerent talk" against enemies.

Trump spent much of his speech panning President Barack Obama's handling of crises in the Middle East, saying the current administration is leaving a legacy of "weakness, confusion and disarray."

"Our foreign policy is a complete and total disaster. No vision, no purpose, no direction, no strategy," he said, identifying as a problem U.S. interventions in Iraq and Libya.

"We've made the Middle East more unstable and chaotic than ever before," Trump said. He singled out Democratic front-runner Hillary Clinton's handling of the deadly attacks on a U.S. compound in Benghazi, Libya, and repeatedly linked the former secretary of state to what he sees as the failures of the Obama administration to show strength around the world.

Even before Trump's address, Clinton's campaign sent out a news release saying the Republican leader has used "the most reckless rhetoric of any major presidential candidate in modern history."

Trump didn't spare former Republican President George W. Bush, criticizing his efforts to take democracy to the Middle East and reaffirming his own opposition to the unpopular Iraq war. Drawing a contrast with some Republicans, Trump said "war and aggression will not be my first instinct," and he pledged to deploy combat forces only as a last resort.

"Our friends and enemies must know that if I draw a line in the sand, I will enforce that line in the sand -- believe me," Trump said. "However, unlike other candidates for the presidency, foreign aggression will not be my first instinct. You cannot have a foreign policy without diplomacy. A superpower understands that caution and restraint are really, truly signs of strength."

He also promised to eradicate the Islamic State but said the campaign against extremism -- or "radical Islam" -- is as much a philosophical struggle as a military one.

"We're getting out of the nation-building business and instead focusing on creating stability on the world," Trump said.

"The Trump administration will lead a free world that is properly armed and funded -- and funded beautifully," he said.

Trump also assailed the Iran nuclear deal, which Clinton supports, and said the U.S. has faced "humiliation" at the hands of Iran. "Iran cannot be allowed to have a nuclear weapon," he said. Obama has "treated Iran with tender love and care and made it a great power," he said.

Stressing a link between immigration and U.S. security, Trump insisted that "we must stop importing extremism through senseless immigration policies." He said "we're also going to have to change our trade, immigration and economic policies to make our economy strong again and put Americans first again."

Trump spoke to an invited audience of conservative-leaning national security experts, as well as some prominent foreign-policy writers -- a largely inside-the-Beltway crowd that included Sen. Jeff Sessions of Alabama, a senior policy adviser to the campaign; a handful of House members; and Grover Norquist, president of Americans for Tax Reform.

Trump's advisers cast Wednesday's speech as the first in a series of policy addresses aimed at calming the nerves of Americans who worry that the businessman doesn't have the experience to handle the range of responsibilities that land on a president's desk. A speech on trade is expected soon.

Information for this article was contributed by Jill Colvin, Julie Pace and staff members of The Associated Press; by Mark Landler and Ashley Parker of The New York Times; and by Sahil Kapur of Bloomberg News.

A Section on 04/28/2016