US 'enmities' towards Iran will not end post-Trump: Khamenei
His remarks came shortly after President Hassan Rouhani said that Iran was "very happy" that Trump was leaving the White House.
Khamenei reiterated his position that Iran should bolster itself to "nullify" sanctions imposed by the Trump administration, but should "not delay" in the event they can be lifted.
"You witnessed what Trump's America and Obama's America did to you," he was quoted as saying on his official website.
"Enmities are not limited to Trump's America and will not end just because he has left office," Khamenei added, addressing the family of top Iranian general Qassem Soleimani, who was killed in a January US air strike in neighbouring Iraq.
"Do not trust the enemy, this is my explicit advice."
Tensions between Tehran and Washington soared during Trump's presidency, especially after 2018, when he pulled Washington out of a landmark nuclear deal and reimposed punishing unilateral sanctions.
This January, Trump ordered a strike near Baghdad airport which killed senior Iranian commander Qassem Soleimani and drew retaliatory Iranian strikes targeting US troops in Iraq.
Khamenei called the missile strikes on US troops a "hard slap" and said "Soleimani's killer" and others involved "must pay at any possible time".
Khamenei's meeting with the slain general's family was his first public engagement since an official close to his office moved to squash rumours about his health last week.
A video released by Khamenei's office showed him walking from behind a curtain to his chair while greeting the family, as well as the Revolutionary Guards' chief Hossein Salami and parliament speaker Mohammad-Bagher Ghalibaf, among other officials.
President-elect Joe Biden, who defeated Trump at the ballot box in November, has signalled a willingness to return to diplomacy with Iran, but Khamenei has on previous occasions cautioned against hopes of an opening with the West.
President Rouhani said his country was happy to see Trump go.
"Some say, 'you are overexcited by the advent of Mr. Biden'. No, we are not, but we are very happy to see Trump leave," he said in televised comments at a cabinet meeting.
"Thank God, these are his final days," Rouhani added, calling Trump a "tyrant", "the most unruly, lawless president" and a "terrorist and murderer".
Trump "creates obstacles for us purchasing (Covid-19) vaccines, (that is) how much this person is bereft of all ethical and human principles," Rouhani said.
Iran is the Middle Eastern country hardest hit by the coronavirus pandemic with over 52,800 deaths from more than 1.1 million cases, according to official figures.
Medicine and other humanitarian goods are supposed to be exempt from US sanctions but in practice few if any banks are willing to risk processing Iranian transactions for fear of incurring heavy penalties in the US courts.
Rouhani's government has repeatedly signalled its openness to the incoming US administration and called on Washington to return to the nuclear deal and lift sanctions.
The electoral college confirmed Biden as the next US president on Monday even as the incumbent continues to refuse to accept defeat.
The formal handover of power will take place on January 20, when Biden is sworn in.
Khamenei underlined that if sanctions "can be lifted, we should not delay it for even an hour".
But he warned: "Do not rely on (others') promises to solve the people's problems (and) do not forget enmities.