NSTask or NSThread?

I have some code attached to NSTimer. About 5 times per second, it interacts with another application (by emulating keystrokes) and, when necessary, issues an NSNotification, which is processed by another piece of code.

While the timer code is running, the UI is unresponsive, so I cannot enable the Stop button that stops the timer.

How can I do it? A separate process (NSTask right?) Or a thread? Remember that at unpredictable times, the timer code will have to send some information to be processed.

Thanks.