(Reuters) – Major U.S. telecom service providers said on Thursday some of their users traveling overseas were having connectivity problems due to an issue at a partner network provider.

In statements to Reuters, Verizon, AT&T and T-Mobile US said they were working to resolve the issue but did not provide details on the extent of the outage or the services partner responsible for it.

However, a person familiar with the matter said it was due to a glitch at Syniverse, a telecom services firm that works with global carriers to support international roaming.

In a response to Reuters, Syniverse said its systems were facing “intermittent connectivity issues” and was working with its network partners to fix the problem.

Verizon said 70% of its customers’ calls and data connections were currently going through, while it worked to resolve the issue.

AT&T said its network was operating normally and some customers traveling overseas were experiencing disruptions “due to an issue outside the AT&T network”.

(Reporting by Yuvraj Malik and Jaspreet Singh in Bengaluru; Editing by Shreya Biswas)