Apple has denied knowing about an alleged attempt by the National Security Agency to create a secret “backdoor” for hacking into the iPhone, as the latest round of disclosures about US internet surveillance forced some of the biggest US tech concerns on to the defensive.

The denial, issued on Tuesday, followed the release of documents showing what were claimed to be NSA attempts to break into a wide range of computer and networking hardware. These included servers made by Dell and Hewlett-Packard, network gear made by Cisco, -Huawei and Juniper, and hard drives from Samsung and -Seagate.

Although dating in most cases from 2008 and involving machines that have been superseded by newer models, the documents pointed to the breadth of the NSA’s efforts to hack into widely used technology. The documents were released in connection with a report in the German publication Der Spiegel.