Iranian Foreign Minister Mohammad Javad Zarif has warned the United States that its days in the region are "numbered", describing the US assassination of a top Iranian military commander last week as "an act of war".In an interview with Al Jazeera on Tuesday, Zarif said his country's response to the killing of Qassem Soleimani, the head of Iran's elite Quds Force, would come in due course."Iran will respond because there was an act of war - an act of war combined with an act of terrorism against a senior official of the Islamic Republic of Iran and a citizen of Iran," Zarif said."We are bound to protect our citizens and our military officials. It was an act that has to be reciprocated by Iran. We will make the necessary deliberations and it will be an act that we will do, not in a hurry, not in a hasty manner."Soleimani was killed along with Abu Mahdi al-Muhandis, the deputy commander of Iraq's Popular Mobilisation Forces (PMF, or Hashd al-Shaabi), an Iran-backed umbrella organisation comprising several militias, and several other people.The attacks triggered a dramatic escalation of tensions in the region, and marked the most significant confrontation between the two countries in recent years."The act by the United States … it enraged the feelings of many people outside Iran, inside Iran - that will have consequences for the United States," Zarif said.His remarks came as dozens of people were killed in a stampede that erupted in the city of Kerman, Soleimani's hometown, during the slain commander's funeral procession.Hundreds of thousands of mourners had gathered in the small city for the burial of Soleimani, which was postponed until further notice following the stampede, according to the semi-official ISNA news agency.Tuesday's funeral comes after days of processions that attracted huge crowds in the streets of Ahvaz in southwestern Iran, followed by Mashhad in the northwest, the capital Tehran and the holy city of Qom.'What are they afraid of?'In response to the assassination of Soleimani, Iran's Supreme Leader Ayatollah Ali Khamenei warned earlier this week that "a harsh retaliation is waiting".Washington argues it killed the commander in self-defence, aiming to disrupt plans to attack US personnel and interests. US President Donald Trump has defended the killing of Soleimani and threatened more retaliatory actions if Iran targets US citizens or assets.Zarif meanwhile was scheduled to attend United Nations meetings later this week, but said his visa request was blocked by Washington."Secretary Pompeo called Secretary General of the United Naions yesterday, and said they didn't have time to consider my request and therefore they will not issue a visa," Zarid said, adding that the decision was in violation of US commitments."What are they afraid of? What do they think would happen if I go to New York," Zarif said."They think that I cannot communicate with the American people without coming to the United States. I can communicate with the American people sitting at home".Although it is unclear how or when Iran may respond to the US assassination of Soleimani, any response is likely to come once the mourning period ends.In a speech broadcast live on television earlier on Tuesday, Zarif said US President Donald Trump committed a "stupid mistake by assassinating the greatest commander who stood in the face of terrorism"."Our region, because of the US' intervention … has become victim to the endless war," Zarif said."Removing the US from western Asia is what will … end wars and death in this region."