Apple has promised to deliver a software development kit (SDK) for the iPhone by February next year.

Chief executive Steve Jobs said in an open letter to customers that the SDK will allow developers to create third-party applications that run directly on the iPhone.

"We want native third-party applications on the iPhone, and we plan to have an SDK in developers' hands in February," he wrote.

"We are excited about creating a vibrant third-party developer community around the iPhone and enabling hundreds of new applications for our users."

Currently, Apple officially allows developers to create iPhone applications that run only through the Safari web browser.

Some developers have created native applications, but they require the iPhone to be unlocked which can cause issues with firmware upgrades.

Apple had originally decided to allow only web-based applications because of security concerns, a point touched on by Jobs in his letter.

"Some claim that viruses and malware are not a problem on mobile phones. This is simply not true," he wrote.

Jobs also echoed the sentiments of those in the security industry who had suggested that the iPhone's use of a desktop-style operating system could actually make it more vulnerable than other mobile phones.

"As our phones become more powerful, these malicious programs will become more dangerous. And since the iPhone is the most advanced phone ever, it will be a highly visible target," he wrote.

Jobs added that, for these reasons, Apple would need until February to perfect the SDK.

Apple may also adopt a developer authentication system similar to the one used by Nokia in which developers must have a traceable signature included in all third-party software.

"We think a few months of patience now will be rewarded by many years of great third-party applications running on safe and reliable iPhones," Jobs concluded.