GLEN, United States (AFP) - Constant engagement is the best option when it comes to confronting Russian President Vladimir Putin, Democratic presidential candidate Hillary Clinton said Saturday, adding that "we have to be much smarter."

Clinton, a former secretary of state who is making another tilt at the White House, also told supporters in Glen, New Hampshire, of her personal experience with Putin.

"We have to be much smarter in how we deal with Putin and how we deal with his ambitions," she said.

"I've dealt with him. I know him. He's not an easy man.

"But I don't think there is any substitute other than constant engagement."

Relations between Moscow and Washington are at their lowest ebb since the Cold War over Russia's actions in neighbouring Ukraine. Moscow denies backing pro-Russian rebels in the east of the country.

Despite the tense ties, Putin sent a message earlier to US counterpart Barack Obama to mark Independence Day in the United States, saying that relations between Moscow and Washington were key to ensuring global stability, and calling for dialogue based on "equality and respect."

Clinton, the frontrunner for the Democratic nomination in the 2016 election, also touched on Iran, which is thrashing out the fine details of a deal with world powers over its nuclear ambitions.

She warned, however, that even if an accord is sealed, "Iran's aggressiveness will not end." "They will continue to be the principle state sponsor of terrorism," Clinton said.

"They will continue to destabilise governments in the region and beyond.

"They will continue to use their proxies like Hezbollah. And they will continue to be an existential threat to Israel."