Da Nang, Vietnam (CNN) President Donald Trump and Russian President Vladimir Putin will not hold a formal meeting at the Asia-Pacific Economic Cooperation summit here in Vietnam, White House press secretary Sarah Sanders has told reporters. But the two world leaders did informally speak a couple times during the two-day meeting.

Trump and Putin were seen casually talking while walking to a photo-op at the Asian-Pacific Rim summit on Saturday. Flanked by sizable entourages, the two spoke for much of the walk down to a vista and stood next to each other for a photo.

The chat, while brief, looked friendly, with both leaders speaking with their hands.

Any interaction between the two world leaders is closely watched given the swirl of investigations into Russia's meddling in the 2016 election, an issue that has plagued the Trump administration for months. Special counsel Robert Mueller's investigation issued his first indictments last week, bringing charges against former top Trump campaign aides Paul Manafort and Rick Gates , who have pleaded not guilty.

Trump and Putin also shook hands and spoke briefly before an APEC plenary session on Saturday and the two world leaders briefly met during a so-called class photo on Friday night, where all the heads of state come together to take a photo before the summit officially starts. The two shook hands and briefly spoke before the photo was snapped. Both wore the oversized, blue traditional-style Vietnamese shirt provided by the host country and stood next to one another for the picture.

US President Donald Trump shakes hands with Russia's President Vladimir Putin as they pose for a group photo ahead of the Asia-Pacific Economic Cooperation (APEC) Summit leaders gala dinner in the central Vietnamese city of Danang on November 10, 2017.

Citing "scheduling conflicts on both sides," though, Sanders said Friday no formal meeting will take place during the two-day gathering. She did leave the door open to informal interactions between the two, a notion reinforced by her Russian counterparts.

"Regarding a Putin meeting, there was never a meeting confirmed, and there will not be one that takes place due to scheduling conflicts on both sides," Sanders said Friday. "There is no formal meeting or anything scheduled for them."

She added: "Now, they're going to be in the same place. Are they going to bump into each other and say hello? Certainly possible, and likely. But in terms of a scheduled, formal meeting, there's not one on the calendar and we don't anticipate that there will be one."

Sanders' statement contradicted what the Russian side told reporters for the last two days, adding to the drama around how Trump and Putin will interact at the international summit.

"The meeting will take place on the sidelines," Putin spokesman Dmitry Peskov said Friday after Sanders' statement, adding that some encounter will take place "one way or another."

The possibility of a formal Trump-Putin meeting hung over the economic summit.

Trump has long denied any collusion between his campaign and Russian operatives during the 2016 election. But US intelligence agencies concluded in a report released earlier this year that Russia ordered an "influence campaign" to harm Hillary Clinton's chances of winning the election.

The delicateness of the Putin-Trump meeting was laid bare on Thursday -- before Sanders' statement -- when the two sides gave dramatically different accounts on whether the two leaders would even meet in the first place.

Russian presidential aide Yuri Ushakov told state-run news organization Itar Tass that Putin and Trump would meet on the sidelines of the Asia-Pacific summit on Friday, adding that the two sides had already agreed to the time and place.

While briefing reporters in China, though, Secretary of State Rex Tillerson declined to say whether Putin and Trump would meet in Vietnam, telling reporters that officials are working to determine whether the two world leaders have sufficient areas of substance to discuss.

"When the two leaders meet, is there something sufficiently substantive?" Tillerson said. "No conclusion has been made on that."

Given the intense focus on any interactions between Trump, his associates and Russian operatives, the meeting -- particularly what is said and not said -- could overshadow the focus of Trump's trip to Vietnam, where the President is set to discuss business, trade and security with an array of Asian leaders.

One cause for concern on the American side is how Trump and Putin's first meeting on the sidelines of the G20 summit in Germany caused a diplomatic dust-up over whether Trump accepted Putin's assurances there was no Russian involvement in the 2016 American election.

Trump opened his first sessions with Putin by "raising the concerns of the American people regarding Russian interference in the 2016 election," Tillerson told reporters after the bilateral meeting. "The President pressed President Putin on more than one occasion regarding Russian involvement. President Putin denied such involvement, as I think he has in the past."

Tillerson didn't say whether Trump accepted his denial. But while Tillerson was briefing US media, Russian Foreign Minister Sergey Lavrov told reporters that Trump accepted Putin's denial.

The charge was immediately denied by a senior Trump administration official, but the turnabout signaled that any diplomatic engagement between the two countries would be complicated and fraught.

Aboard Air Force One as he kicked off his five-country, 13-day trip throughout Asia, Trump told reporters that he "expected" to meet with Putin on the visit because the United States hopes to enlist Russia in the fight against North Korea.

"We want Putin's help on North Korea," he said.

The first half of Trump's trip through Asia has primarily focused on how to deal with the rogue nation. Trump gave a direct and bellicose speech in South Korea about Pyongyang, at one point speaking directly to North Korean leader Kim Jong Un.

"North Korea is not the paradise your grandfather envisioned," he said. "It is a hell that no person deserves."

Trump will continue to focus on North Korea in Vietnam, senior White House officials tell CNN, but his time at the summit will primarily focus on free trade.

"The President's engagements at APEC will reinforce the US commitment to an equitable, sustainable and rules-based international economic system based on market principles," one official said.