Rix (LIV) reconstructs an acrodynamic (Narten) present *sēk-. (In the Leiden school view of Derksen and Kortlandt, this would not yield the required acute vowel.)
Kortlandt asserts that the vowel of *sěkti was lengthened to disambiguate the verb from the root preserved in Lithuanian sèkti(“to watch, to follow”), Proto-Slavic *sočiti(“to pursue, to indicate”) < Proto-Indo-European *sekʷ-.
Derksen notes that the short root vowel is preserved in секыра(sekyra, “axe”).
Vasmer (Fasmer), Max (Maks) (1964–1973), “секу”, in Etimologičeskij slovarʹ russkovo jazyka [Etymological Dictionary of the Russian Language] (in Russian), translated from German and supplemented by Trubačóv Oleg, Moscow: Progress
Černyx, P. Ja. (1999), “сечь”, in Istoriko-etimologičeskij slovarʹ russkovo jazyka [Historical-Etymological Dictionary of the Russian Language] (in Russian), volume 2, 3rd reprint edition, Moscow: Russkij jazyk, page 159