China has rebuffed repeated U.S. offers to share advanced vaccines as Beijing battles a fast-spreading wave of COVID-19, a rejection that’s led to growing frustration among American officials concerned about a resurgence of the pandemic.

Worried about the rise of new variants and impact on China’s economy, the U.S. has repeatedly offered mRNA vaccines and other assistance to President Xi Jinping’s government through private channels, according to U.S. officials who asked not to be identified discussing the deliberations.

U.S. officials have also proposed indirect ways to supply the vaccines in an effort to accommodate political sensitivities in China on accepting foreign aid, they said, without providing more details.