ISLAMABAD, Pakistan—The nuclear arms race between India and Pakistan is intensifying, with new weaponry and more aggressive doctrines that are stoking tensions between two powers at growing risk of confrontation.

The neighbors, which have fought three wars and many skirmishes, have in recent months adopted dueling steps aimed at gaining strategic advantage. Each has more than 100 nuclear warheads and new ways to deliver them from land, air and sea. India appears to be considering changing its nuclear doctrine to allow a first strike against Pakistan, according to some analysts.

Among rival developments, India tested interceptor missiles twice this year as part of its plan to develop a ballistic missile-defense shield. Pakistan in January tested a missile with multiple warheads capable of evading it.

India said last year it began testing its first homemade nuclear-powered submarine at sea and a nuclear missile capable of striking all of Pakistani territory from far offshore. Then Pakistan this year said it had tested its own undersea nuclear missile capable of carrying out a retaliatory strike.

India’s army chief said for the first time this year that it devised a plan for a rapid, shallow, conventional invasion of Pakistan that some analysts say could be unleashed in response to a cross-border terror attack like the Mumbai assault of 2008, which killed 166 people.