The Vatican’s diplomatic presence in Hong Kong – and the Church as a whole – is coming under mounting pressure as Beijing moves to extinguish opposition voices in the city under a new national security law.

In May, two Chinese nuns who work at the mission were arrested by mainland authorities during a visit home to Hebei province. The nuns, in their 40s, were detained for three weeks before being released to house arrest without being charged. They are forbidden to leave the mainland. Meanwhile, Western diplomats say, Chinese security agents have stepped up surveillance of the Holy See’s mission in recent months.

The arrests, which haven’t been previously reported, are viewed by top clerics in China and in the Vatican as a sign Beijing wants the mission shut. It lacks official standing because the Holy See and China haven’t established formal diplomatic ties. While priests are sometimes arrested on the mainland, “it is highly unusual for nuns to be detained,” said another of the clerics, who has long-time contacts on the mainland. “Normally they are left alone.”