Burns and Jake Sullivan led a secret diplomatic channel to Tehran which eventually facilitated the Iran nuclear deal.
"Terrorists murdered an eminent Iranian scientist today," Iran's Foreign Minister Javad Zarif said.
"The new U.S. administration should reconsider its inhumane behaviors toward other countries," Iranian President Hassan Rouhani said.
Secretary of State Mike Pompeo said Saturday the U.S. had reimposed international sanctions on Iran, but American allies disputed the announcement.
Trump withdrew from the Joint Comprehensive Plan of Action in 2018, claiming the deal was too lenient and vowing a re-negotiated agreement.
European diplomat Helga Maria Schmid said signatories aim to "ensure full implementation of the agreement despite current challenges."
"We have made great progress in the field of strategic and deterrent weaponry," Iranian President Hassan Rouhani said.
Days after the United States lost a bid to extend a United Nations arms embargo on Iran, China and Russia agreed to a summit aimed at avoiding potential conflict between the U.S. and Tehran.
Iranian President Hassan Rouhani specifically urged China and Russia to resist the American demand to extend restrictions.
U.N. sanctions may ultimately be reimposed on the nation.
"Instead of caving, Iran is moving step by step out of the nuclear deal and in recent months it hit six ships, attacked Saudi oil facilities and set fire to the U.S. embassy in Baghdad," Chris Wallace pointed out.
"This moment right now is on Barack Obama, not Donald Trump," Fox News contributor Pete Hegseth said.
The Obama administration sanctioned Soleimani in 2011 over his involvement in a plan to assassinate a Saudi diplomat in Washington, D.C.
Reza Pahlavi, the exiled heir to the now-defunct Iranian throne, tells Newsweek that the bloody unrest in his home country can only be solved by removing the regime in Tehran.
Trump may see a deal with Kim as his best chance for a major victory. But fallout from our domestic politics is so toxic it might scuttle the agreement—no matter its terms. This would be a massive mistake.
"These contradictory remarks are either a sign of their confused policies or is a trick to confuse the other side," Ali Khamenei said Tuesday.
Hamad bin Jassim bin Jabor al-Thani said Iranians won't crack under American economic pressure, and will look to play the long game instead.
Russian foreign minister Sergey Lavrov said American pressure on the Iran nuclear deal is designed to create a pretense for conflict.
"It's not news that the U.S. and UK differ in how to ensure Iran is never able to acquire a nuclear weapon," a spokesman for the UK Foreign Office said.
"Iranian people have proved over the past 40 years that they are ready to pay for safeguarding national identity, territorial integrity, freedom, independence and Islamic establishment," Masoumeh Ebtekar said.
"The U.S. has destabilized security of the international system through unilateralism and extra-terrestrial sanctions," Ali Shamkhani said.
"The entire world recognizes the U.S. as a country reneging on its promises," President Hassan Rouhani said on Tuesday.
Enriched uranium is required to produce nuclear power, but at a higher level of enrichment it can also be used to build nuclear weapons.
"The Iranians should understand very clearly that we will continue to take actions that deter Iran from engaging in this kind of behavior," Secretary of State Mike Pompeo said.
"That's a very serious issue if they continue to do that," Trump's national security adviser warned.
"We are actually considering escalating with Iran in a war that would further destabilize the region and unleash God knows what in terms of chaos in a very troubled region already," Elise Jordan said.
Washington has been at odds with Europe over Iran since the Trump administration abandoned the 2015 nuclear deal.
"They have all shown an interest in dragging the United States into a conflict," Javad Zarif told Fox News Sunday. "I do not believe that President Trump wants to do that. I believe that President Trump ran on a campaign promise of not bringing the United States into another war."
"The rest of the world sees perfectly well that Washington's policy is getting increasingly aggressive and reckless," a Russian Foreign Ministry statement said.