How did the fugitive slave act and the Kansas-Nebraska act increase tensions between the north and the south?

The Fugitive Slave Act allowed the appointment of official slave-catchers to arrest runaways. This aroused indignation among Northern abolitionists, and promoted support for the Underground Railroad - the system of safe-houses where runaways could be sumggled into Canada. Harriet Beecher Stowe was so angered by it that she wrote 'Uncle Tom's Cabin', which engaged the attention of large numbers who had not been especially interested up till then.

The Kansas-Nebraska Act was an attempt to put the slavery question to a local vote, for each territory wanting to join the USA. It had the effect of encouraging cross-border terrorists to intimidate voters and claim that all the results were rigged.