Mohsen Fakhrizadeh, a top Iranian nuclear scientist, was shot dead near Tehran on November 27. The killing has sparked outrage in the country, with key leadership figures announcing they would retaliate.

There is speculation about who carried out the attack. In addition to suspicions, the killing has raised regional tensions, and also brought new uncertainties to Iran-U.S. relations and the Joint Comprehensive Plan of Action, or the Iran nuclear deal. The deal was reached in 2015 between Iran, the five permanent members of the UN Security Council—China, France, the U.S., UK and Russia—plus Germany and the EU. However, the U.S. announced its exit from it in 2018 under President Donald Trump and since then Iran-U.S. tensions have been rising.

Over the past four decades, their relations have often shifted, either toward confrontation or detente. The killing not only damages Iran’s nuclear capabilities, but possibly also increases obstacles to a thaw in relations after Joe Biden takes over as U.S. president in January 2021.

Since August 2018, Trump has been tightening sanctions against Iran. By September 16, Washington had imposed more than 30 rounds of sanctions on 970 individuals and entities, according to a report released by the U.S. Department of State. The reasons range from alleged support to terrorism and development of ballistic missiles to human rights violations and cyber attacks against the U.S. and its allies.

Even if the Biden administration decides to rejoin the Iran nuclear deal, it will be hard to lift so many sanctions.

Though the U.S. failed to cut Iranian oil exports to zero, its sanctions have led to a sharp decline in Iranian oil production and revenue. The Iranian Government has been earning far less and in an effort to tackle the deficit, has been printing more money, leading to devaluation and inflation. Affected by the decline in foreign exchange earnings and difficulties in importing raw materials, enterprises in Iran are operating under capacity, and the employment rate has dropped significantly.

U.S. sanctions have also impaired Iran’s fight against the novel coronavirus disease. One of the major reasons why the number of infections and deaths has remained high in Iran is the severe shortage of medical resources to treat patients.

Even though humanitarian supplies such as medicine, medical equipment and food are not targeted, risks of trade settlement have undermined the willingness of other countries to trade with Iran, leaving it with inadequate medical supplies.

Biden’s election has raised hopes among Iranians for a thaw though their leadership stresses that the result of the election will not affect the policy toward Washington. Iranians believe Biden will abandon Trump’s policy of extreme pressure on their country, resuming former U.S. President Barack Obama’s policy to push forward negotiations by promising to ease some sanctions.

But a detente is unlikely to happen soon. Biden would have many domestic issues to deal with. Also, lifting the sanctions introduced by the Trump administration would require congressional approval and can’t be achieved overnight. Biden might also put limits on Iran’s development of ballistic missiles and regional expansion in the negotiations, which Iran may find difficult to accept.

Unlike Trump, Biden is a supporter of the Iran nuclear deal. During his election campaign, he criticized Trump’s withdrawal from it as damaging to U.S. national interests and isolating the U.S. internationally, promising to return to the deal if Iran would stick to it.

Iran’s parliament passed a bill on November 29, aiming to revive its nuclear program after the assassination. The bill would suspend UN inspections of its nuclear facilities and boost its uranium enrichment program if international sanctions on the oil and gas sector are not lifted.

The U.S. and Iran don’t trust each other and each wants the other to make the first move. Biden has insisted that the U.S. would not rejoin the nuclear deal and lift sanctions until Iran complied with it. He will seek to extend the duration of restrictions on Iran’s production of fissile material that could be used to make a nuclear bomb and to limit its “malign” regional activities. Iran is demanding the sanctions be lifted before it returns to the negotiating table.

The EU, which opposes Trump’s withdrawal from the nuclear deal and the resumption of unilateral sanctions, remains committed to it but shares the U.S. desire to limit Iran’s missile capabilities and regional influence. EU countries are likely to join the U.S. in pressuring Iran once the Iranian Government implements the new bill.

China has condemned the killing of the Iranian scientist. It stands for safeguarding the international nuclear non-proliferation system and opposes unilateral sanctions, while maintaining normal exchanges and cooperation with Iran.

The Chinese suggestion is to establish a multilateral platform for the Middle East and the Gulf region to resolve concerns through equal-footed consultation.

The author is a researcher with the Institute of West Asian and African Studies, Chinese Academy of Social Sciences.