Donald Trump has warned that Hillary Clinton's plan for Syria would 'lead to World War Three,' because of the potential for conflict with military forces from nuclear-armed Russia.

In an interview on foreign policy, Trump said defeating ISIS is a higher priority than persuading Syrian President Bashar al-Assad to step down - despite US policy which opposes both Assad and Islamic State.

Trump questioned how Clinton would negotiate with Vladimir Putin after demonizing him, suggesting she could drag the US into a major conflict.

Hillary Clinton has won the support of many conservatives, who support her foreign policy stance. They suggest that if elected president in November, she will escalate tensions with Russia and take a tougher stance on the Middle East.

Donald Trump claims that if Hillary Clinton, pictured with husband Bill, wins the presidency it could lead to World War Three

Clinton has called for the establishment of a no-fly zone in Syria and 'safe zones' on the ground to protect non-combatants. Some analysts fear that protecting those zones could bring the United States into direct conflict with Russian fighter jets.

'What we should do is focus on ISIS. We should not be focusing on Syria,' Trump told Reuters as he dined on fried eggs and sausage at his Trump National Doral golf resort.

'You're going to end up in World War Three over Syria if we listen to Hillary Clinton.

'You're not fighting Syria any more, you're fighting Syria, Russia and Iran, all right? Russia is a nuclear country, but a country where the nukes work as opposed to other countries that talk,' he said.

Trump said Assad is much stronger now than he was three years ago and said getting Assad to leave power was less important than defeating Islamic State.

'Assad is secondary, to me, to ISIS,' he said.

President Bashar al-Assad, left, has supported Russia's bombing campaign, which Putin, right has used as a display of the country's military might

He also criticized Clinton's handling of US-Russian relations while Secretary of State and said her harsh criticism of Putin raised questions about 'how she is going to go back and negotiate with this man who she has made to be so evil' if she wins the presidency.

Clinton's campaign dismissed the criticism, noting that both Republican and Democratic national security experts have denounced Trump as unfit to be commander-in-chief.

'Once again, he is parroting Putin's talking points and playing to Americans' fears, all while refusing to lay out any plans of his own for defeating ISIS or alleviating humanitarian suffering in Syria,' Clinton spokesman Jesse Lehrich said in a statement.

Trump also bemoaned a lack of Republican unity behind his candidacy, saying he would easily win the election if the party leaders would support him.

'If we had party unity, we couldn't lose this election to Hillary Clinton,' he said.

Syrian government forces patrol near Aleppo's thermal power plant after they took control of the area on the eastern outskirts of Syria's northern embattled city from Islamic State. Trump has said that getting Assad to leave power was less important than defeating Islamic State

Trump has said Clinton could drag the United States into a world war with a more aggressive posture toward resolving the conflict

Trump also blamed Barack Obama for a downturn in US relations with the Philippines under its new president, Rodrigo Duterte, saying the president 'wants to focus on his golf game' rather than engage with world leaders.

Since assuming office, Duterte has expressed open hostility toward the United States, rejecting criticism of his violent anti-drug clampdown, calling Obama 'son of a b****' and telling the United States not to treat his country 'like a dog with a leash.'

The Obama administration has expressed optimism that the two countries can remain firm allies.

Trump said Duterte's latest comments showed 'a lack of respect for our country.'