Engineering code specifies that the surface width of cracks should not generally exceed 0.3 mm. Anything, however generally any cracks from 0.3mm to 3mm is not critical. If it is more than 3mm then immediate action need to be taken.

Take picture so that can see how severe is the crack.

3mm is the width of the crack not length of the crack ... anyway dun worry too much if it is less than 3mm width ... if i remember LDP highway had >30mm width cracks and still standing ... however they had rectify the structure already ... cheers

It might be structural crack or not as some crack do occur due to method of construction ....
Since it is just 1mm crack then it can be considered a hairline crack which most of the time caused by :

Drying shrinkage – as concrete cures and goes through it’s chemical reaction between the water and cement particles it begins to “dry” out. As with most materials that dry out they begin to shrink causing hairline cracks to appear.

Thermal shrinkage – all materials shrink and expand with changes in temperature. As concrete cools down (the chemical reaction that’s created when water and cement is mixed creates heat) it shrinks. The shrinking from cooling down can also cause hairline cracks to appear.

If you are still worried then you can monitor at the concrete on both sides of the crack and try to determine whether or not there has been any relative movement between the sides, up/down, left/right, in/out. Like what I said earlier on you need to take immediate action if the cracks are more than 3mm width.