When President Trump likely meets with Russian President Vladimir Putin during a summit in Vietnam, all eyes will be on how they interact and whether they can agree on contentious issues.

Trump and Putin could meet Friday or Saturday in Da Nang on the sidelines of the Asia-Pacific Economic Cooperation (APEC) conference.   

Aboard Air Force One flying to Vietnam, press secretary Sarah Huckabee Sanders told reporters no formal meeting is planned.

"Now, they’re going to be in the same place," she said. "Are they going to bump into each other and say hello? Certainly possible and likely."

Even before leaving last week for his 12-day Asia trip, Trump emphasized the significance of meeting with Putin, telling Fox News: "Putin is very important because they can help us with North Korea. They can help us with Syria. We have to talk about Ukraine.”

But any meeting comes amid an ongoing U.S. investigation of alleged collusion last year between Russia and Trump's presidential campaign. So watch for any signs of awkward body language when they are together in public. The two leaders met for the first time in July at the G-20 summit in Hamburg.

Mike McFaul, a former U.S. ambassador to Russia who used to arrange such meetings for President Barack Obama, noted that Trump has a more casual style than professional politicians and openly praised Putin in the past.

"Considering the situation at home, I don't think it helps for him to talk to Putin that way," McFaul said. "He should focus on substance and more professional diplomatic engagement instead of this schmoozing."

Steven Pifer, who advised former presidents Bill Clinton and George W. Bush on Russia and Ukraine, said Trump should insist on a formal meeting with Putin. 

And Trump could look bad if he meets with Putin "and nothing comes out of it," said Pifer, who's now at the Brookings Institution.

Here are several topics the two leaders could discuss: 

North Korea

Trump, speaking in South Korea on Tuesday, said all countries, including Russia, need to do their part to isolate North Korea to pressure leader Kim Jong Un to abandon his nuclear weapons and missile programs. The reclusive nation has tested six nuclear devices, its most powerful one on Sept. 3. North Korea has also launched 23 missile tests since Trump took office, including an intercontinental ballistic missile on July 4 capable of reaching the U.S. mainland. 

Trump in Seoul also called on all nations, “including China and Russia,” to fully implement United Nations sanctions on North Korea, to downgrade diplomatic relations with the North and to “sever all ties of trade and technology.” 

President Trump

The U.S. wants Russia and others to stop using thousands of North Korean laborers — Trump called them slaves — in construction and timber industries, providing important revenue for Pyongyang’s nuclear weapons and missiles programs. The U.S. is also concerned that as China cuts back on trade with North Korea, Russia is picking up some of the slack.

Asked about Russia's position on the issue, Kremlin spokesman Dmitry Peskov said, "Let us wait until our heads of state hold contacts," the official Tass Russian News Agency reported.

Iran nuclear deal

Trump wants to strengthen or renegotiate parts of the deal to limit Iran’s nuclear activities in return for the lifting of international sanctions, which was negotiated by the U.S., China, France, Germany, Russia and the United Kingdom during the Obama administration.

Putin has pledged to remain in the Iran deal, and Peskov has warned of “very negative consequences” for regional security and nuclear proliferation efforts if the U.S. pulls out of the agreement. 

Countering Iranian influence

Trump has vowed to curtail what he calls Iran’s destabilizing activities in the Middle East, especially in Syria and Lebanon. Russia’s military support for Syrian President Bashar Assad in Syria's lengthy civil war means Putin could exert some influence on Iran, which has given Assad military advisers and thousands of fighters. The recent resignation of Lebanon's prime minister, Saad Hariri, leaves that country's Iran-backed Hezbollah in a stronger position.

With much of Syria now under the control of forces loyal to Assad, Russia is considering a partial drawdown of its forces, according to Tass.

Jon Huntsman, the U.S. ambassador to Russia, met Wednesday with Russian Deputy Foreign Minister Gennady Gatilov to discuss a possible settlement in Syria. Other topics included "resistance to terrorism” and political settlements on the Korean peninsula and in eastern Ukraine, the Russian Foreign Ministry said.

U.S. sanctions on Russia over Ukraine and the 2016 election

The Trump administration has been slow to implement sanctions that Congress  imposed on Russia for supporting anti-government forces in Ukraine and for Moscow's alleged role in the 2016 presidential election. While Trump denies that Russians played any part in his victory, the delay means the president could have some leverage to pressure Putin. 

Putin has proposed a U.N. peacekeeping force to reduce the violence in the former Soviet republic, where 10,000 Ukrainians have died since 2014. He said peacekeepers need to protect Russian-speaking people in the Donbas region of eastern Ukraine, where much of the fighting occurred.

Kurt Volker, the U.S. special representative for Ukraine negotiations, last week called the Russian proposal worth considering, but pointed out that peacekeepers are needed in the entire eastern Ukraine region, not just where the fighting is.

U.S. and European sanctions against Russia are costing Putin's inner circle, so the peacekeeping proposal might give the Russian president "a dignified way out," said Anders Fogh Rasmussen, the former NATO chief who is now foreign policy adviser to Ukraine President Petro Poroshenko. 

More: Analysis: War with China? Trump, U.S. seek to avoid collision course

More: Trump practiced Art of the Deal in China. Analysts say it may not solve long-term challenges

More: Let's 'make a deal': Trump moderates tone on North Korea, at least for now

More: Trump warns North Korea not to 'try us' with nuclear weapons

Contributing: David Jackson