MTN Group Ltd. said efforts to repatriate about 200 million euros ($237 million) from its Iranian unit will become tougher after U.S. President Donald Trump reinstated economic sanctions on the Islamic republic.

Bloomberg reports that Africa’s biggest wireless carrier by sales had more than $1 billion stuck in Iran due to U.S.-led sanctions that ended following a landmark agreement in 2015. Since that accord, the Johannesburg-based company has been steadily withdrawing the funds.

MTN shares fell as much as 4 percent on Wednesday, the most in more than a month, on concerns that cash would be stuck in Iran. The shares recovered to trade 2.7 percent lower at 121.55 rand as of 9:48 a.m. in Johannesburg.

“We will continue to monitor the situation including the response of the Iranian authorities,” MTN said in a statement. The company has repatriated 88 million euros to date in 2018. In March, the wireless operator said it expected the last of the cash would be received by September.

Iran is MTN’s second-largest market after Nigeria with 43 million customers at the end of 2017. It has operated in the country since 2005.