WASHINGTON (AP) — Iran has increased its offensive cyberattacks against the U.S. government and critical infrastructure as tensions have grown between the two nations, cybersecurity firms say.

In recent weeks, hackers believed to be working for the Iranian government have targeted U.S. government agencies, as well as sectors of the economy, including oil and gas, sending waves of spear-phishing emails, according to representatives of cybersecurity companies CrowdStrike and FireEye, which regularly track such activity.

It was not known if any of the hackers managed to gain access to the targete...