The Latvian form UG-uns is primordial since Latvian UD-ens = "water".
The Indo-European root *AS is a more "modern" version.

Comparable words are  English ASH
Latvian DEG or DEDZ "burn".
Latvian ASHs "sharp, pungent".

Related roots can also be seen in the reverse term
"to put out a fire"
in Hebrew D ' K and Latvian DZES

There is also good correspondence in other "fire"-words.

Hebrew 'OR and Latvian KUR "light up a fire, burn"

Hebrew B'R and  Latvian PIRts "sauna" PEL-nis "ash"
BRuce "burn wound" and English BURn

Hebrew SUT and Latvian SUT "steam, heat food, stew, braise"

Hebrew KWH, KEWIJA and Latvian KVE-le "glow, heat" KVE-pi "soot"

Hebrew SARAB and Latvian SARAVA (SARAUT)
which means "pulled  together" and
whence in this sense "burned, the scarred skin" and,
since Latvian MIESA means "body"
there is Hebrew MISHRAFOR i.e. MIESU-SARAVOT (SARAUT)
with a root in SAR(T)- found in Hebrew as ZAR-
Hebrew LQT and Latvian LIESmOT German LOH "blaze"

In Hebrew 'ESH 'OKELA

the latter term is Latvian UOGLE "coal(s), burning substance"